libgfs2: Replace lgfs2_rgrps_root() with lgfs2_attach_rgrps()
    
    We only use this function to attach an rgrp tree to the sbd in order to
    call functions that (for now) expect it to be attached, so provide a
    more opaque function to do that.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>