Sparse reports a warning at xfs_ail_check()

warning: context imbalance in xfs_ail_check() - unexpected unlock

The root cause is the missing annotation at xfs_ail_check()

Add the missing __must_hold(&ailp->ail_lock) annotation
Signed-off-by: NJules Irenge <jbi.octave@gmail.com>
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>