Update, if_vether(4) dedicated MPI IO processing refactored.

Any MPI, which bypasses protocol stack by netgraph(4) subsystem
is annotated by

	/*
	 * Annotate MPI by M_PROTO2 flag, if ouput interface
	 * type denotes IFT_VETHER.
	 */
	if (ifp->if_type == IFT_VETHER)
		m->m_flags |= M_PROTO2;

during runtime of ng_ether_rcv_lower() in ng_ether(4) and finally
processed by

static void
vether_start_locked(struct vether_softc	*sc, struct ifnet *ifp)
{
	struct mbuf *m;
	int error;

	VETHER_LOCK_ASSERT(sc);

	ifp->if_drv_flags |= IFF_DRV_OACTIVE;
	for (;;) {
		IFQ_DEQUEUE(&ifp->if_snd, m);
		if (m == NULL)
			break;

 		if (ifp->if_bridge == NULL) {
			m_freem(m);
			continue;
		}
	/*
	 * MPIs are passed protocol stack or processed
	 * by netgraph(4) subsystem must contain PCI.
	 */
		if ((m->m_flags & M_PKTHDR) == 0) {
			m_freem(m);
			continue;
		}
		ETHER_BPF_MTAP(ifp, m);

		if (m->m_pkthdr.rcvif == NULL) {
	/*
	 * Any frame which is not injected by netgraph(4)
	 * is transmitted by hooked bridge_output() in
	 * ether_output().
	 */
			if ((m->m_flags & M_PROTO2) == 0) {
				m_freem(m);
				continue;
			}
			m->m_pkthdr.rcvif = ifp;
			m->m_flags &= ~M_PROTO2;
			BRIDGE_OUTPUT(ifp, m, error);
		} else if (m->m_pkthdr.rcvif != ifp) {
	/*
	 * Frames received by other interfaces and are
	 * enqueued by bridge_output() are processed
	 * by ether_input().
	 */
			m->m_pkthdr.rcvif = ifp;
			(*ifp->if_input)(ifp, m);
		} else {
	/*
	 * Duplicated MPIs are being discarded.
	 */
			m_freem(m);
			continue;
		}
	}
	ifp->if_drv_flags &= ~IFF_DRV_OACTIVE;
}

Causes, such that any other by ether_output() transmitted frame will be
discarded, iff instance is not member of some instance of if_bridge(4)
and transmitted MPI did not bypassed protocol stack by ng_ether(4) or
MPI was duplicated.