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_dir2_data.c b/fs/xfs/xfs_dir2_data.c
index 70acff4..1952f00 100644
--- a/fs/xfs/xfs_dir2_data.c
+++ b/fs/xfs/xfs_dir2_data.c
@@ -268,8 +268,7 @@
 	struct xfs_mount	*mp = bp->b_target->bt_mount;
 
 	if ((xfs_sb_version_hascrc(&mp->m_sb) &&
-	     !xfs_verify_cksum(bp->b_addr, BBTOB(bp->b_length),
-					  XFS_DIR3_DATA_CRC_OFF)) ||
+	     !xfs_buf_verify_cksum(bp, XFS_DIR3_DATA_CRC_OFF)) ||
 	    !xfs_dir3_data_verify(bp)) {
 		XFS_CORRUPTION_ERROR(__func__, XFS_ERRLEVEL_LOW, mp, bp->b_addr);
 		xfs_buf_ioerror(bp, EFSCORRUPTED);