xfs: don't assert fail with AIL lock held

Been hitting AIL ordering assert failures recently, but been unable
to trace them down because the system immediately hangs up onteh
spinlock that was held when this assert fires:

XFS: Assertion failed: XFS_LSN_CMP(prev_lip->li_lsn, lip->li_lsn) <= 0, file: fs/xfs/xfs_trans_ail.c, line: 52

Move the assertions outside of the spinlock so the corpse can
be dissected. Thanks to Brian Foster for supplying a clean
way of doing this.

Signed-Off-By: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>