ip_tunnel: push generic protocol handling to ip_tunnel module.

Process skb tunnel header before sending packet to protocol handler.
this allows code sharing between gre and ovs gre modules.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
index 76bb8de..6cee844 100644
--- a/net/ipv6/sit.c
+++ b/net/ipv6/sit.c
@@ -640,9 +640,14 @@
 
 static int ipip_rcv(struct sk_buff *skb)
 {
-	const struct iphdr *iph = ip_hdr(skb);
+	const struct iphdr *iph;
 	struct ip_tunnel *tunnel;
 
+	if (iptunnel_pull_header(skb, 0, tpi.proto))
+		goto drop;
+
+	iph = ip_hdr(skb);
+
 	tunnel = ipip6_tunnel_lookup(dev_net(skb->dev), skb->dev,
 				     iph->saddr, iph->daddr);
 	if (tunnel != NULL) {