Fix uaf in xfs_trans_ail_delete during xlog force shutdown.
In commit cd6f79d1 ("xfs: run callbacks before waking waiters in
xlog_state_shutdown_callbacks") changed the order of running callbacks
and wait for iclog completion to avoid unmount path untimely destroy AIL.
But which seems not enough to ensue this, adding mdelay in
`xfs_buf_item_unpin` can prove that.

The reproduction is as follows. To ensure destroy AIL safely,
we should wait all xlog ioend workers done and sync the AIL.

==================================================================
BUG: KASAN: use-after-free in xfs_trans_ail_delete+0x240/0x2a0
Read of size 8 at addr ffff888023169400 by task kworker/1:1H/43

CPU: 1 PID: 43 Comm: kworker/1:1H Tainted: G        W
6.1.0-rc1-00002-gc28266863c4a #137
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.13.0-1ubuntu1.1 04/01/2014
Workqueue: xfs-log/sda xlog_ioend_work
Call Trace:
 <TASK>
 dump_stack_lvl+0x4d/0x66
 print_report+0x171/0x4a6
 kasan_report+0xb3/0x130
 xfs_trans_ail_delete+0x240/0x2a0
 xfs_buf_item_done+0x7b/0xa0
 xfs_buf_ioend+0x1e9/0x11f0
 xfs_buf_item_unpin+0x4c8/0x860
 xfs_trans_committed_bulk+0x4c2/0x7c0
 xlog_cil_committed+0xab6/0xfb0
 xlog_cil_process_committed+0x117/0x1e0
 xlog_state_shutdown_callbacks+0x208/0x440
 xlog_force_shutdown+0x1b3/0x3a0
 xlog_ioend_work+0xef/0x1d0
 process_one_work+0x6f9/0xf70
 worker_thread+0x578/0xf30
 kthread+0x28c/0x330
 ret_from_fork+0x1f/0x30
 </TASK>

Allocated by task 9606:
 kasan_save_stack+0x1e/0x40
 kasan_set_track+0x21/0x30
 __kasan_kmalloc+0x7a/0x90
 __kmalloc+0x59/0x140
 kmem_alloc+0xb2/0x2f0
 xfs_trans_ail_init+0x20/0x320
 xfs_log_mount+0x37e/0x690
 xfs_mountfs+0xe36/0x1b40
 xfs_fs_fill_super+0xc5c/0x1a70
 get_tree_bdev+0x3c5/0x6c0
 vfs_get_tree+0x85/0x250
 path_mount+0xec3/0x1830
 do_mount+0xef/0x110
 __x64_sys_mount+0x150/0x1f0
 do_syscall_64+0x35/0x80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

Freed by task 9662:
 kasan_save_stack+0x1e/0x40
 kasan_set_track+0x21/0x30
 kasan_save_free_info+0x2a/0x40
 __kasan_slab_free+0x105/0x1a0
 __kmem_cache_free+0x99/0x2d0
 kvfree+0x3a/0x40
 xfs_log_unmount+0x60/0xf0
 xfs_unmountfs+0xf3/0x1d0
 xfs_fs_put_super+0x78/0x300
 generic_shutdown_super+0x151/0x400
 kill_block_super+0x9a/0xe0
 deactivate_locked_super+0x82/0xe0
 deactivate_super+0x91/0xb0
 cleanup_mnt+0x32a/0x4a0
 task_work_run+0x15f/0x240
 exit_to_user_mode_prepare+0x188/0x190
 syscall_exit_to_user_mode+0x12/0x30
 do_syscall_64+0x42/0x80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

The buggy address belongs to the object at ffff888023169400
 which belongs to the cache kmalloc-128 of size 128
The buggy address is located 0 bytes inside of
 128-byte region [ffff888023169400, ffff888023169480)

The buggy address belongs to the physical page:
page:ffffea00008c5a00 refcount:1 mapcount:0 mapping:0000000000000000
index:0xffff888023168f80 pfn:0x23168
head:ffffea00008c5a00 order:1 compound_mapcount:0 compound_pincount:0
flags: 0x1fffff80010200(slab|head|node=0|zone=1|lastcpupid=0x1fffff)
raw: 001fffff80010200 ffffea00006b3988 ffffea0000577a88 ffff88800f842ac0
raw: ffff888023168f80 0000000000150007 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff888023169300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff888023169380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff888023169400: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff888023169480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff888023169500: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================
Disabling lock debugging due to kernel taint

Fixes: cd6f79d1 ("xfs: run callbacks before waking waiters in xlog_state_shutdown_callbacks")
Signed-off-by: Guo Xuenan <guoxuenan@huawei.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>