gfs2: cleanup: call gfs2_rgrp_ondisk2lvb from gfs2_rgrp_out

Before this patch gfs2_rgrp_ondisk2lvb was called after every call
to gfs2_rgrp_out. This patch just calls it directly from within
gfs2_rgrp_out, and moves the function to be before it so we don't
need a function prototype.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>