net/ipv6/sit.c: return unhandled skb to tunnel4_rcv

I found a problem using an IPv6 over IPv4 tunnel.  When CONFIG_IPV6_SIT
was enabled, the packets would be rejected as net/ipv6/sit.c was catching
all IPPROTO_IPV6 packets and returning an ICMP port unreachable error.

I think this patch fixes the problem cleanly.  I believe the code in
net/ipv4/tunnel4.c:tunnel4_rcv takes care of it properly if none of the
handlers claim the skb.

Signed-off-by: David McCullough <david_mccullough@mcafee.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -606,8 +606,9 @@ static int ipip6_rcv(struct sk_buff *skb)
 		return 0;
 	}
 
-	icmp_send(skb, ICMP_DEST_UNREACH, ICMP_PORT_UNREACH, 0);
+	/* no tunnel matched,  let upstream know, ipsec may handle it */
 	rcu_read_unlock();
+	return 1;
 out:
 	kfree_skb(skb);
 	return 0;