gfs2: Pass resource group to rgblk_free

Function rgblk_free can only deal with one resource group at a time, so
pass that resource group is as a parameter.  Several of the callers
already have the resource group at hand, so we only need additional
lookup code in a few places.

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