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>

fs/xfs/xfs_sb.c

@@ -610,8 +610,7 @@ xfs_sb_read_verify(
 						XFS_SB_VERSION_5) ||
 	     dsb->sb_crc != 0)) {
 
-		if (!xfs_verify_cksum(bp->b_addr, BBTOB(bp->b_length),
-				      XFS_SB_CRC_OFF)) {
+		if (!xfs_buf_verify_cksum(bp, XFS_SB_CRC_OFF)) {
 			/* Only fail bad secondaries on a known V5 filesystem */
 			if (bp->b_bn == XFS_SB_DADDR ||
 			    xfs_sb_version_hascrc(&mp->m_sb)) {