Lockdep with fstests test case btrfs/041 detected a unsafe locking
scenario when we allocate the log node on a zoned filesystem.

btrfs/041
 ============================================
 WARNING: possible recursive locking detected
 5.11.0-rc7+ #939 Not tainted
 --------------------------------------------
 xfs_io/698 is trying to acquire lock:
 ffff88810cd673a0 (&root->log_mutex){+.+.}-{3:3}, at: btrfs_sync_log+0x3d1/0xee0 [btrfs]

 but task is already holding lock:
 ffff88810b0fc3a0 (&root->log_mutex){+.+.}-{3:3}, at: btrfs_sync_log+0x313/0xee0 [btrfs]

 other info that might help us debug this:
  Possible unsafe locking scenario:

        CPU0
        ----
   lock(&root->log_mutex);
   lock(&root->log_mutex);

  *** DEADLOCK ***

  May be due to missing lock nesting notation

 2 locks held by xfs_io/698:
  #0: ffff88810cd66620 (sb_internal){.+.+}-{0:0}, at: btrfs_sync_file+0x2c3/0x570 [btrfs]
  #1: ffff88810b0fc3a0 (&root->log_mutex){+.+.}-{3:3}, at: btrfs_sync_log+0x313/0xee0 [btrfs]

 stack backtrace:
 CPU: 0 PID: 698 Comm: xfs_io Not tainted 5.11.0-rc7+ #939
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4-rebuilt.opensuse.org 04/01/2014
 Call Trace:
  dump_stack+0x77/0x97
  __lock_acquire.cold+0xb9/0x32a
  lock_acquire+0xb5/0x400
  ? btrfs_sync_log+0x3d1/0xee0 [btrfs]
  __mutex_lock+0x7b/0x8d0
  ? btrfs_sync_log+0x3d1/0xee0 [btrfs]
  ? btrfs_sync_log+0x3d1/0xee0 [btrfs]
  ? find_first_extent_bit+0x9f/0x100 [btrfs]
  ? __mutex_unlock_slowpath+0x35/0x270
  btrfs_sync_log+0x3d1/0xee0 [btrfs]
  btrfs_sync_file+0x3a8/0x570 [btrfs]
  __x64_sys_fsync+0x34/0x60
  do_syscall_64+0x33/0x40
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

This happens, because we are taking the ->log_mutex albeit it has already
been locked.

Also while at it, fix the bogus unlock of the tree_log_mutex in the error
handling.

Fixes: 3ddebf27 ("btrfs: zoned: reorder log node allocation on zoned filesystem")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>