Before relocating a block group we pause scrub, then do the relocation and
then unpause scrub. The relocation process requires starting and committing
a transaction, and if we have a failure in the critical section of the
transaction commit path (transaction state >= TRANS_STATE_COMMIT_START),
we will deadlock if there is a paused scrub.

That results in stack traces like the following:

  [42.479] BTRFS info (device sdc): relocating block group 53876686848 flags metadata|raid6
  [42.936] BTRFS warning (device sdc): Skipping commit of aborted transaction.
  [42.936] ------------[ cut here ]------------
  [42.936] BTRFS: Transaction aborted (error -28)
  [42.936] WARNING: CPU: 11 PID: 346822 at fs/btrfs/transaction.c:1977 btrfs_commit_transaction+0xcc8/0xeb0 [btrfs]
  [42.936] Modules linked in: dm_flakey dm_mod loop btrfs (...)
  [42.936] CPU: 11 PID: 346822 Comm: btrfs Tainted: G        W          6.3.0-rc2-btrfs-next-127+ #1
  [42.936] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
  [42.936] RIP: 0010:btrfs_commit_transaction+0xcc8/0xeb0 [btrfs]
  [42.936] Code: ff ff 45 8b (...)
  [42.936] RSP: 0018:ffffb58649633b48 EFLAGS: 00010282
  [42.936] RAX: 0000000000000000 RBX: ffff8be6ef4d5bd8 RCX: 0000000000000000
  [42.936] RDX: 0000000000000002 RSI: ffffffffb35e7782 RDI: 00000000ffffffff
  [42.936] RBP: ffff8be6ef4d5c98 R08: 0000000000000000 R09: ffffb586496339e8
  [42.936] R10: 0000000000000001 R11: 0000000000000001 R12: ffff8be6d38c7c00
  [42.936] R13: 00000000ffffffe4 R14: ffff8be6c268c000 R15: ffff8be6ef4d5cf0
  [42.936] FS:  00007f381a82b340(0000) GS:ffff8beddfcc0000(0000) knlGS:0000000000000000
  [42.936] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [42.936] CR2: 00007f1e35fb7638 CR3: 0000000117680006 CR4: 0000000000370ee0
  [42.936] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  [42.936] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  [42.936] Call Trace:
  [42.936]  <TASK>
  [42.936]  ? start_transaction+0xcb/0x610 [btrfs]
  [42.936]  prepare_to_relocate+0x111/0x1a0 [btrfs]
  [42.936]  relocate_block_group+0x57/0x5d0 [btrfs]
  [42.936]  ? btrfs_wait_nocow_writers+0x25/0xb0 [btrfs]
  [42.936]  btrfs_relocate_block_group+0x248/0x3c0 [btrfs]
  [42.936]  ? __pfx_autoremove_wake_function+0x10/0x10
  [42.936]  btrfs_relocate_chunk+0x3b/0x150 [btrfs]
  [42.936]  btrfs_balance+0x8ff/0x11d0 [btrfs]
  [42.936]  ? __kmem_cache_alloc_node+0x14a/0x410
  [42.936]  btrfs_ioctl+0x2334/0x32c0 [btrfs]
  [42.937]  ? mod_objcg_state+0xd2/0x360
  [42.937]  ? refill_obj_stock+0xb0/0x160
  [42.937]  ? seq_release+0x25/0x30
  [42.937]  ? __rseq_handle_notify_resume+0x3b5/0x4b0
  [42.937]  ? percpu_counter_add_batch+0x2e/0xa0
  [42.937]  ? __x64_sys_ioctl+0x88/0xc0
  [42.937]  __x64_sys_ioctl+0x88/0xc0
  [42.937]  do_syscall_64+0x38/0x90
  [42.937]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
  [42.937] RIP: 0033:0x7f381a6ffe9b
  [42.937] Code: 00 48 89 44 24 (...)
  [42.937] RSP: 002b:00007ffd45ecf060 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
  [42.937] RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007f381a6ffe9b
  [42.937] RDX: 00007ffd45ecf150 RSI: 00000000c4009420 RDI: 0000000000000003
  [42.937] RBP: 0000000000000003 R08: 0000000000000013 R09: 0000000000000000
  [42.937] R10: 00007f381a60c878 R11: 0000000000000246 R12: 00007ffd45ed0423
  [42.937] R13: 00007ffd45ecf150 R14: 0000000000000000 R15: 00007ffd45ecf148
  [42.937]  </TASK>
  [42.937] ---[ end trace 0000000000000000 ]---
  [42.937] BTRFS: error (device sdc: state A) in cleanup_transaction:1977: errno=-28 No space left
  [59.196] INFO: task btrfs:346772 blocked for more than 120 seconds.
  [59.196]       Tainted: G        W          6.3.0-rc2-btrfs-next-127+ #1
  [59.196] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  [59.196] task:btrfs           state:D stack:0     pid:346772 ppid:1      flags:0x00004002
  [59.196] Call Trace:
  [59.196]  <TASK>
  [59.196]  __schedule+0x392/0xa70
  [59.196]  ? __pv_queued_spin_lock_slowpath+0x165/0x370
  [59.196]  schedule+0x5d/0xd0
  [59.196]  __scrub_blocked_if_needed+0x74/0xc0 [btrfs]
  [59.197]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.197]  scrub_pause_off+0x21/0x50 [btrfs]
  [59.197]  scrub_simple_mirror+0x1c7/0x950 [btrfs]
  [59.197]  ? scrub_parity_put+0x1a5/0x1d0 [btrfs]
  [59.198]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.198]  scrub_stripe+0x20d/0x740 [btrfs]
  [59.198]  scrub_chunk+0xc4/0x130 [btrfs]
  [59.198]  scrub_enumerate_chunks+0x3e4/0x7a0 [btrfs]
  [59.198]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.198]  btrfs_scrub_dev+0x236/0x6a0 [btrfs]
  [59.199]  ? btrfs_ioctl+0xd97/0x32c0 [btrfs]
  [59.199]  ? _copy_from_user+0x7b/0x80
  [59.199]  btrfs_ioctl+0xde1/0x32c0 [btrfs]
  [59.199]  ? refill_stock+0x33/0x50
  [59.199]  ? should_failslab+0xa/0x20
  [59.199]  ? kmem_cache_alloc_node+0x151/0x460
  [59.199]  ? alloc_io_context+0x1b/0x80
  [59.199]  ? preempt_count_add+0x70/0xa0
  [59.199]  ? __x64_sys_ioctl+0x88/0xc0
  [59.199]  __x64_sys_ioctl+0x88/0xc0
  [59.199]  do_syscall_64+0x38/0x90
  [59.199]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
  [59.199] RIP: 0033:0x7f82ffaffe9b
  [59.199] RSP: 002b:00007f82ff9fcc50 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
  [59.199] RAX: ffffffffffffffda RBX: 000055b191e36310 RCX: 00007f82ffaffe9b
  [59.199] RDX: 000055b191e36310 RSI: 00000000c400941b RDI: 0000000000000003
  [59.199] RBP: 0000000000000000 R08: 00007fff1575016f R09: 0000000000000000
  [59.199] R10: 0000000000000000 R11: 0000000000000246 R12: 00007f82ff9fd640
  [59.199] R13: 000000000000006b R14: 00007f82ffa87580 R15: 0000000000000000
  [59.199]  </TASK>
  [59.199] INFO: task btrfs:346773 blocked for more than 120 seconds.
  [59.200]       Tainted: G        W          6.3.0-rc2-btrfs-next-127+ #1
  [59.200] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  [59.201] task:btrfs           state:D stack:0     pid:346773 ppid:1      flags:0x00004002
  [59.201] Call Trace:
  [59.201]  <TASK>
  [59.201]  __schedule+0x392/0xa70
  [59.201]  ? __pv_queued_spin_lock_slowpath+0x165/0x370
  [59.201]  schedule+0x5d/0xd0
  [59.201]  __scrub_blocked_if_needed+0x74/0xc0 [btrfs]
  [59.201]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.201]  scrub_pause_off+0x21/0x50 [btrfs]
  [59.202]  scrub_simple_mirror+0x1c7/0x950 [btrfs]
  [59.202]  ? scrub_parity_put+0x1a5/0x1d0 [btrfs]
  [59.202]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.202]  scrub_stripe+0x20d/0x740 [btrfs]
  [59.202]  scrub_chunk+0xc4/0x130 [btrfs]
  [59.203]  scrub_enumerate_chunks+0x3e4/0x7a0 [btrfs]
  [59.203]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.203]  btrfs_scrub_dev+0x236/0x6a0 [btrfs]
  [59.203]  ? btrfs_ioctl+0xd97/0x32c0 [btrfs]
  [59.203]  ? _copy_from_user+0x7b/0x80
  [59.203]  btrfs_ioctl+0xde1/0x32c0 [btrfs]
  [59.204]  ? should_failslab+0xa/0x20
  [59.204]  ? kmem_cache_alloc_node+0x151/0x460
  [59.204]  ? alloc_io_context+0x1b/0x80
  [59.204]  ? preempt_count_add+0x70/0xa0
  [59.204]  ? __x64_sys_ioctl+0x88/0xc0
  [59.204]  __x64_sys_ioctl+0x88/0xc0
  [59.204]  do_syscall_64+0x38/0x90
  [59.204]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
  [59.204] RIP: 0033:0x7f82ffaffe9b
  [59.204] RSP: 002b:00007f82ff1fbc50 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
  [59.204] RAX: ffffffffffffffda RBX: 000055b191e36790 RCX: 00007f82ffaffe9b
  [59.204] RDX: 000055b191e36790 RSI: 00000000c400941b RDI: 0000000000000003
  [59.204] RBP: 0000000000000000 R08: 00007fff1575016f R09: 0000000000000000
  [59.204] R10: 0000000000000000 R11: 0000000000000246 R12: 00007f82ff1fc640
  [59.204] R13: 000000000000006b R14: 00007f82ffa87580 R15: 0000000000000000
  [59.204]  </TASK>
  [59.204] INFO: task btrfs:346774 blocked for more than 120 seconds.
  [59.205]       Tainted: G        W          6.3.0-rc2-btrfs-next-127+ #1
  [59.205] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  [59.206] task:btrfs           state:D stack:0     pid:346774 ppid:1      flags:0x00004002
  [59.206] Call Trace:
  [59.206]  <TASK>
  [59.206]  __schedule+0x392/0xa70
  [59.206]  schedule+0x5d/0xd0
  [59.206]  __scrub_blocked_if_needed+0x74/0xc0 [btrfs]
  [59.206]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.206]  scrub_pause_off+0x21/0x50 [btrfs]
  [59.207]  scrub_simple_mirror+0x1c7/0x950 [btrfs]
  [59.207]  ? scrub_parity_put+0x1a5/0x1d0 [btrfs]
  [59.207]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.207]  scrub_stripe+0x20d/0x740 [btrfs]
  [59.208]  scrub_chunk+0xc4/0x130 [btrfs]
  [59.208]  scrub_enumerate_chunks+0x3e4/0x7a0 [btrfs]
  [59.208]  ? __mutex_unlock_slowpath.isra.0+0x9a/0x120
  [59.208]  btrfs_scrub_dev+0x236/0x6a0 [btrfs]
  [59.208]  ? btrfs_ioctl+0xd97/0x32c0 [btrfs]
  [59.209]  ? _copy_from_user+0x7b/0x80
  [59.209]  btrfs_ioctl+0xde1/0x32c0 [btrfs]
  [59.209]  ? should_failslab+0xa/0x20
  [59.209]  ? kmem_cache_alloc_node+0x151/0x460
  [59.209]  ? alloc_io_context+0x1b/0x80
  [59.209]  ? preempt_count_add+0x70/0xa0
  [59.209]  ? __x64_sys_ioctl+0x88/0xc0
  [59.209]  __x64_sys_ioctl+0x88/0xc0
  [59.209]  do_syscall_64+0x38/0x90
  [59.209]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
  [59.209] RIP: 0033:0x7f82ffaffe9b
  [59.209] RSP: 002b:00007f82fe9fac50 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
  [59.209] RAX: ffffffffffffffda RBX: 000055b191e36c10 RCX: 00007f82ffaffe9b
  [59.209] RDX: 000055b191e36c10 RSI: 00000000c400941b RDI: 0000000000000003
  [59.209] RBP: 0000000000000000 R08: 00007fff1575016f R09: 0000000000000000
  [59.209] R10: 0000000000000000 R11: 0000000000000246 R12: 00007f82fe9fb640
  [59.209] R13: 000000000000006b R14: 00007f82ffa87580 R15: 0000000000000000
  [59.209]  </TASK>
  [59.209] INFO: task btrfs:346775 blocked for more than 120 seconds.
  [59.210]       Tainted: G        W          6.3.0-rc2-btrfs-next-127+ #1
  [59.210] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  [59.211] task:btrfs           state:D stack:0     pid:346775 ppid:1      flags:0x00004002
  [59.211] Call Trace:
  [59.211]  <TASK>
  [59.211]  __schedule+0x392/0xa70
  [59.211]  schedule+0x5d/0xd0
  [59.211]  __scrub_blocked_if_needed+0x74/0xc0 [btrfs]
  [59.211]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.211]  scrub_pause_off+0x21/0x50 [btrfs]
  [59.212]  scrub_simple_mirror+0x1c7/0x950 [btrfs]
  [59.212]  ? scrub_parity_put+0x1a5/0x1d0 [btrfs]
  [59.212]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.212]  scrub_stripe+0x20d/0x740 [btrfs]
  [59.213]  scrub_chunk+0xc4/0x130 [btrfs]
  [59.213]  scrub_enumerate_chunks+0x3e4/0x7a0 [btrfs]
  [59.213]  ? __mutex_unlock_slowpath.isra.0+0x9a/0x120
  [59.213]  btrfs_scrub_dev+0x236/0x6a0 [btrfs]
  [59.213]  ? btrfs_ioctl+0xd97/0x32c0 [btrfs]
  [59.214]  ? _copy_from_user+0x7b/0x80
  [59.214]  btrfs_ioctl+0xde1/0x32c0 [btrfs]
  [59.214]  ? should_failslab+0xa/0x20
  [59.214]  ? kmem_cache_alloc_node+0x151/0x460
  [59.214]  ? alloc_io_context+0x1b/0x80
  [59.214]  ? preempt_count_add+0x70/0xa0
  [59.214]  ? __x64_sys_ioctl+0x88/0xc0
  [59.214]  __x64_sys_ioctl+0x88/0xc0
  [59.214]  do_syscall_64+0x38/0x90
  [59.214]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
  [59.214] RIP: 0033:0x7f82ffaffe9b
  [59.214] RSP: 002b:00007f82fe1f9c50 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
  [59.214] RAX: ffffffffffffffda RBX: 000055b191e37090 RCX: 00007f82ffaffe9b
  [59.214] RDX: 000055b191e37090 RSI: 00000000c400941b RDI: 0000000000000003
  [59.214] RBP: 0000000000000000 R08: 00007fff1575016f R09: 0000000000000000
  [59.214] R10: 0000000000000000 R11: 0000000000000246 R12: 00007f82fe1fa640
  [59.214] R13: 000000000000006b R14: 00007f82ffa87580 R15: 0000000000000000
  [59.214]  </TASK>
  [59.214] INFO: task btrfs:346776 blocked for more than 120 seconds.
  [59.215]       Tainted: G        W          6.3.0-rc2-btrfs-next-127+ #1
  [59.216] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  [59.217] task:btrfs           state:D stack:0     pid:346776 ppid:1      flags:0x00004002
  [59.217] Call Trace:
  [59.217]  <TASK>
  [59.217]  __schedule+0x392/0xa70
  [59.217]  ? __pv_queued_spin_lock_slowpath+0x165/0x370
  [59.217]  schedule+0x5d/0xd0
  [59.217]  __scrub_blocked_if_needed+0x74/0xc0 [btrfs]
  [59.217]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.217]  scrub_pause_off+0x21/0x50 [btrfs]
  [59.217]  scrub_simple_mirror+0x1c7/0x950 [btrfs]
  [59.217]  ? scrub_parity_put+0x1a5/0x1d0 [btrfs]
  [59.218]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.218]  scrub_stripe+0x20d/0x740 [btrfs]
  [59.218]  scrub_chunk+0xc4/0x130 [btrfs]
  [59.218]  scrub_enumerate_chunks+0x3e4/0x7a0 [btrfs]
  [59.219]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.219]  btrfs_scrub_dev+0x236/0x6a0 [btrfs]
  [59.219]  ? btrfs_ioctl+0xd97/0x32c0 [btrfs]
  [59.219]  ? _copy_from_user+0x7b/0x80
  [59.219]  btrfs_ioctl+0xde1/0x32c0 [btrfs]
  [59.219]  ? should_failslab+0xa/0x20
  [59.219]  ? kmem_cache_alloc_node+0x151/0x460
  [59.219]  ? alloc_io_context+0x1b/0x80
  [59.219]  ? preempt_count_add+0x70/0xa0
  [59.219]  ? __x64_sys_ioctl+0x88/0xc0
  [59.219]  __x64_sys_ioctl+0x88/0xc0
  [59.219]  do_syscall_64+0x38/0x90
  [59.219]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
  [59.219] RIP: 0033:0x7f82ffaffe9b
  [59.219] RSP: 002b:00007f82fd9f8c50 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
  [59.219] RAX: ffffffffffffffda RBX: 000055b191e37510 RCX: 00007f82ffaffe9b
  [59.219] RDX: 000055b191e37510 RSI: 00000000c400941b RDI: 0000000000000003
  [59.219] RBP: 0000000000000000 R08: 00007fff1575016f R09: 0000000000000000
  [59.219] R10: 0000000000000000 R11: 0000000000000246 R12: 00007f82fd9f9640
  [59.219] R13: 000000000000006b R14: 00007f82ffa87580 R15: 0000000000000000
  [59.219]  </TASK>
  [59.219] INFO: task btrfs:346822 blocked for more than 120 seconds.
  [59.220]       Tainted: G        W          6.3.0-rc2-btrfs-next-127+ #1
  [59.221] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  [59.222] task:btrfs           state:D stack:0     pid:346822 ppid:1      flags:0x00004002
  [59.222] Call Trace:
  [59.222]  <TASK>
  [59.222]  __schedule+0x392/0xa70
  [59.222]  schedule+0x5d/0xd0
  [59.222]  btrfs_scrub_cancel+0x91/0x100 [btrfs]
  [59.222]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.222]  btrfs_commit_transaction+0x572/0xeb0 [btrfs]
  [59.223]  ? start_transaction+0xcb/0x610 [btrfs]
  [59.223]  prepare_to_relocate+0x111/0x1a0 [btrfs]
  [59.223]  relocate_block_group+0x57/0x5d0 [btrfs]
  [59.223]  ? btrfs_wait_nocow_writers+0x25/0xb0 [btrfs]
  [59.223]  btrfs_relocate_block_group+0x248/0x3c0 [btrfs]
  [59.224]  ? __pfx_autoremove_wake_function+0x10/0x10
  [59.224]  btrfs_relocate_chunk+0x3b/0x150 [btrfs]
  [59.224]  btrfs_balance+0x8ff/0x11d0 [btrfs]
  [59.224]  ? __kmem_cache_alloc_node+0x14a/0x410
  [59.224]  btrfs_ioctl+0x2334/0x32c0 [btrfs]
  [59.225]  ? mod_objcg_state+0xd2/0x360
  [59.225]  ? refill_obj_stock+0xb0/0x160
  [59.225]  ? seq_release+0x25/0x30
  [59.225]  ? __rseq_handle_notify_resume+0x3b5/0x4b0
  [59.225]  ? percpu_counter_add_batch+0x2e/0xa0
  [59.225]  ? __x64_sys_ioctl+0x88/0xc0
  [59.225]  __x64_sys_ioctl+0x88/0xc0
  [59.225]  do_syscall_64+0x38/0x90
  [59.225]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
  [59.225] RIP: 0033:0x7f381a6ffe9b
  [59.225] RSP: 002b:00007ffd45ecf060 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
  [59.225] RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007f381a6ffe9b
  [59.225] RDX: 00007ffd45ecf150 RSI: 00000000c4009420 RDI: 0000000000000003
  [59.225] RBP: 0000000000000003 R08: 0000000000000013 R09: 0000000000000000
  [59.225] R10: 00007f381a60c878 R11: 0000000000000246 R12: 00007ffd45ed0423
  [59.225] R13: 00007ffd45ecf150 R14: 0000000000000000 R15: 00007ffd45ecf148
  [59.225]  </TASK>

What happens is the following:

1) A scrub is running, so fs_info->scrubs_running is 1;

2) Task A starts block group relocation, and at btrfs_relocate_chunk() it
   pauses scrub by calling btrfs_scrub_pause(). That increments
   fs_info->scrub_pause_req from 0 to 1 and waits for the scrub task to
   pause (for fs_info->scrubs_paused to be == to fs_info->scrubs_running);

3) The scrub task pauses at scrub_pause_off(), waiting for
   fs_info->scrub_pause_req to decrease to 0;

4) Task A then enters btrfs_relocate_block_group(), and down that call
   chain we start a transaction and then attempt to commit it;

5) When task A calls btrfs_commit_transaction(), it either will do the
   commit itself or wait for some other task that already started the
   commit of the transaction - it doesn't matter which case;

6) The transaction commit enters state TRANS_STATE_COMMIT_START;

7) An error happens during the transaction commit, like -ENOSPC when
   running delayed refs or delayed items for example;

8) This results in calling transaction.c:cleanup_transaction(), where
   we call btrfs_scrub_cancel(), incrementing fs_info->scrub_cancel_req
   from 0 to 1, and blocking this task waiting for fs_info->scrubs_running
   to decrease to 0;

9) From this point on, both the transaction commit and the scrub task
   hang forever:

   1) The transaction commit is waiting for fs_info->scrubs_running to
      be decreased to 0;

   2) The scrub task is at scrub_pause_off() waiting for
      fs_info->scrub_pause_req to decrease to 0 - so it can not proceed
      to stop the scrub and decrement fs_info->scrubs_running from 0 to 1.

   Therefore resulting in a deadlock.

Fix this by having cleanup_transaction(), called if a transaction commit
fails, not call btrfs_scrub_cancel() if relocation is in progress, and
having btrfs_relocate_block_group() call btrfs_scrub_cancel() instead if
the relocation failed and a transaction abort happened.

This was triggered with btrfs/061 from fstests.

Fixes: 55e3a601 ("btrfs: Fix data checksum error cause by replace with io-load.")
CC: stable@vger.kernel.org # 4.14+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>