xfs: add a XFS_IS_CORRUPT macro

Add a new macro, XFS_IS_CORRUPT, which we will use to integrate some
corruption reporting when the corruption test expression is true.  This
will be used in the next patch to remove the ugly XFS_WANT_CORRUPT*
macros.

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