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/ipv6/udp_offload.c b/net/ipv6/udp_offload.c
index cf05cf0..3bb3a89 100644
--- a/net/ipv6/udp_offload.c
+++ b/net/ipv6/udp_offload.c
@@ -21,6 +21,10 @@
 	const struct ipv6hdr *ipv6h;
 	struct udphdr *uh;
 
+	/* UDP Tunnel offload on ipv6 is not yet supported. */
+	if (skb->encapsulation)
+		return -EINVAL;
+
 	if (!pskb_may_pull(skb, sizeof(*uh)))
 		return -EINVAL;
 
@@ -56,7 +60,9 @@
 		/* Packet is from an untrusted source, reset gso_segs. */
 		int type = skb_shinfo(skb)->gso_type;
 
-		if (unlikely(type & ~(SKB_GSO_UDP | SKB_GSO_DODGY |
+		if (unlikely(type & ~(SKB_GSO_UDP |
+				      SKB_GSO_DODGY |
+				      SKB_GSO_UDP_TUNNEL |
 				      SKB_GSO_GRE) ||
 			     !(type & (SKB_GSO_UDP))))
 			goto out;