tunneling: Add generic Tunnel segmentation.

Adds generic tunneling offloading support for IPv4-UDP based
tunnels.
GSO type is added to request this offload for a skb.
netdev feature NETIF_F_UDP_TUNNEL is added for hardware offloaded
udp-tunnel support. Currently no device supports this feature,
software offload is used.

This can be used by tunneling protocols like VXLAN.

CC: Jesse Gross <jesse@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index dc3f677..9e5882c 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1283,6 +1283,7 @@
 	int ihl;
 	int id;
 	unsigned int offset = 0;
+	bool tunnel;
 
 	if (unlikely(skb_shinfo(skb)->gso_type &
 		     ~(SKB_GSO_TCPV4 |
@@ -1290,6 +1291,7 @@
 		       SKB_GSO_DODGY |
 		       SKB_GSO_TCP_ECN |
 		       SKB_GSO_GRE |
+		       SKB_GSO_UDP_TUNNEL |
 		       0)))
 		goto out;
 
@@ -1304,6 +1306,8 @@
 	if (unlikely(!pskb_may_pull(skb, ihl)))
 		goto out;
 
+	tunnel = !!skb->encapsulation;
+
 	__skb_pull(skb, ihl);
 	skb_reset_transport_header(skb);
 	iph = ip_hdr(skb);
@@ -1323,7 +1327,7 @@
 	skb = segs;
 	do {
 		iph = ip_hdr(skb);
-		if (proto == IPPROTO_UDP) {
+		if (!tunnel && proto == IPPROTO_UDP) {
 			iph->id = htons(id);
 			iph->frag_off = htons(offset >> 3);
 			if (skb->next != NULL)