The interaction between msg_dgm_ref_recv() and msg_dgm_ref_destructor()
doesn't allow two references from messaging_dgm_ref() to be free'd
during the loop in msg_dgm_ref_recv().
In addition to the global 'refs' list, we also need to
have a global 'next_ref' pointer, which can be cleared in
As AD DC we hit this when using irpc in auth_winbind.