xfs: clean up the ISVALID macro in xfs_bmap_adjacent

Turn the  ISVALID macro defined and used inside in xfs_bmap_adjacent
that relies on implict context into a proper inline function.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>