ipv4: use dst with ref during bcast/mcast loopback

Make sure skb dst has reference when moving to
another context. Currently, I don't see protocols that can
hit it when sending broadcasts/multicasts to loopback using
noref dsts, so it is just a precaution.
Signed-off-by: NJulian Anastasov <ja@ssi.bg>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -122,6 +122,7 @@ static int ip_dev_loopback_xmit(struct sk_buff *newskb)
 	newskb->pkt_type = PACKET_LOOPBACK;
 	newskb->ip_summed = CHECKSUM_UNNECESSARY;
 	WARN_ON(!skb_dst(newskb));
+	skb_dst_force(newskb);
 	netif_rx_ni(newskb);
 	return 0;
 }