net: Add GSO support for UDP tunnels with checksum

Added a new netif feature for GSO_UDP_TUNNEL_CSUM. This indicates
that a device is capable of computing the UDP checksum in the
encapsulating header of a UDP tunnel.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -345,6 +345,8 @@ enum {
 	SKB_GSO_UDP_TUNNEL = 1 << 9,
 
 	SKB_GSO_MPLS = 1 << 10,
+
+	SKB_GSO_UDP_TUNNEL_CSUM = 1 << 11,
 };
 
 #if BITS_PER_LONG > 32