gif(4): Assert that gif_output() isn't called for EtherIP

With the current implementation of if_bridge(4), bridge_enqueue()
calls gif_transmit() only. Ensure it stays that way so that the
expectations in both drivers are either met or changed accordingly.

PR:		227450

sys/net/if_gif.c

@@ -407,6 +407,9 @@ gif_output(struct ifnet *ifp, struct mbuf *m, const struct sockaddr *dst,
 {
 	uint32_t af;
 
+	KASSERT(ifp->if_bridge == NULL,
+	    ("%s: unexpectedly called with bridge attached", __func__));
+
 	if (dst->sa_family == AF_UNSPEC)
 		memcpy(&af, dst->sa_data, sizeof(af));
 	else