xfs: add helper for verifying checksums on xfs_bufs

Many/most callers of xfs_verify_cksum() pass bp->b_addr and
BBTOB(bp->b_length) as the first 2 args.  Add a helper
which can just accept the bp and the crc offset, and work
it out on its own, for brevity.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --git a/fs/xfs/xfs_ialloc.c b/fs/xfs/xfs_ialloc.c
index d79210b..d6a879d 100644
--- a/fs/xfs/xfs_ialloc.c
+++ b/fs/xfs/xfs_ialloc.c
@@ -1571,8 +1571,8 @@
 	int		agi_ok = 1;
 
 	if (xfs_sb_version_hascrc(&mp->m_sb))
-		agi_ok = xfs_verify_cksum(bp->b_addr, BBTOB(bp->b_length),
-					  XFS_AGI_CRC_OFF);
+		agi_ok = xfs_buf_verify_cksum(bp, XFS_AGI_CRC_OFF);
+
 	agi_ok = agi_ok && xfs_agi_verify(bp);
 
 	if (unlikely(XFS_TEST_ERROR(!agi_ok, mp, XFS_ERRTAG_IALLOC_READ_AGI,