gfs2: eliminate update_rgrp_lvb_unlinked

Function update_rgrp_lvb_unlinked used to do the same thing as
be32_add_cpu. This patch removes it in favor of using be32_add_cpu
directly.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Reviewed-by: Andrew Price <anprice@redhat.com>