xfs: add helper for updating checksums on xfs_bufs

Many/most callers of xfs_update_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 d6a879d..4657586 100644
--- a/fs/xfs/xfs_ialloc.c
+++ b/fs/xfs/xfs_ialloc.c
@@ -1600,7 +1600,7 @@
 
 	if (bip)
 		XFS_BUF_TO_AGI(bp)->agi_lsn = cpu_to_be64(bip->bli_item.li_lsn);
-	xfs_update_cksum(bp->b_addr, BBTOB(bp->b_length), XFS_AGI_CRC_OFF);
+	xfs_buf_update_cksum(bp, XFS_AGI_CRC_OFF);
 }
 
 const struct xfs_buf_ops xfs_agi_buf_ops = {