xfs: make the ASSERT() condition likely

The ASSERT() condition is the normal case, not the exception,
so testing the condition should be likely(), not unlikely().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>