xfs_bmap_longest_free_extent() can return an error when accessing
the AGF fails. In this case, the behaviour of
xfs_filestream_pick_ag() is conditional on the error. We may
continue the loop, or break out of it. The error handling after the
loop cleans up the perag reference held when the break occurs. If we
continue, the next loop iteration handles cleaning up the perag
reference.

EIther way, we don't need to release the active perag reference when
xfs_bmap_longest_free_extent() fails. Doing so means we do a double
decrement on the active reference count, and this causes tha active
reference count to fall to zero. At this point, new active
references will fail.

This leads to unmount hanging because it tries to grab active
references to that perag, only for it to fail. This happens inside a
loop that retries until a inode tree radix tree tag is cleared,
which cannot happen because we can't get an active reference to the
perag.

The unmount livelocks in this path:

  xfs_reclaim_inodes+0x80/0xc0
  xfs_unmount_flush_inodes+0x5b/0x70
  xfs_unmountfs+0x5b/0x1a0
  xfs_fs_put_super+0x49/0x110
  generic_shutdown_super+0x7c/0x1a0
  kill_block_super+0x27/0x50
  deactivate_locked_super+0x30/0x90
  deactivate_super+0x3c/0x50
  cleanup_mnt+0xc2/0x160
  __cleanup_mnt+0x12/0x20
  task_work_run+0x5e/0xa0
  exit_to_user_mode_prepare+0x1bc/0x1c0
  syscall_exit_to_user_mode+0x16/0x40
  do_syscall_64+0x40/0x80
  entry_SYSCALL_64_after_hwframe+0x63/0xcd
Reported-by: Pengfei Xu <pengfei.xu@intel.com>
Fixes: eb70aa2d ("xfs: use for_each_perag_wrap in xfs_filestream_pick_ag")
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>