ipv6: call iptunnel_xmit with NULL sock pointer if no tunnel sock is available

Fixes: 79b16aad ("udp_tunnel: Pass UDP socket down through udp_tunnel{, 6}_xmit_skb().")
Reported-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -983,7 +983,7 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 
 	skb_set_inner_ipproto(skb, IPPROTO_IPV6);
 
-	err = iptunnel_xmit(skb->sk, rt, skb, fl4.saddr, fl4.daddr,
+	err = iptunnel_xmit(NULL, rt, skb, fl4.saddr, fl4.daddr,
 			    protocol, tos, ttl, df,
 			    !net_eq(tunnel->net, dev_net(dev)));
 	iptunnel_xmit_stats(err, &dev->stats, dev->tstats);