Merge r260188 from head:

Fix regression from r249894. Now we pass "gw" as argument to if_output
  method, thus for multicast case we need it to point at "dst".

PR:		185395

sys/netinet/ip_output.c

@@ -330,6 +330,12 @@ ip_output(struct mbuf *m, struct mbuf *opt, struct route *ro, int flags,
 	    __func__, mtu, rte, (rte != NULL) ? rte->rt_flags : 0, ifp));
 	if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr))) {
 		m->m_flags |= M_MCAST;
+		/*
+		 * IP destination address is multicast.  Make sure "gw"
+		 * still points to the address in "ro".  (It may have been
+		 * changed to point to a gateway address, above.)
+		 */
+		gw = dst;
 		/*
 		 * See if the caller provided any multicast options
 		 */