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>