udp_tunnel: Pass UDP socket down through udp_tunnel{, 6}_xmit_skb().

That was we can make sure the output path of ipv4/ipv6 operate on
the UDP socket rather than whatever random thing happens to be in
skb->sk.

Based upon a patch by Jiri Pirko.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index b6ae959..27470cd 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h
@@ -827,6 +827,7 @@
 int ip6_mc_input(struct sk_buff *skb);
 
 int __ip6_local_out(struct sk_buff *skb);
+int ip6_local_out_sk(struct sock *sk, struct sk_buff *skb);
 int ip6_local_out(struct sk_buff *skb);
 
 /*