kernel-netlink: Enable UDP GRO

This enables GRO offload for inbound ESP-in-UDP packets if the
esp4|6_offload modules are loaded.  Note that inbound ESP or ESP-in-UDP
packets won't be visible on layer 3 in Netfilter or tcpdump.

src/include/linux/udp.h

@@ -29,6 +29,7 @@ struct udphdr {
 /* UDP socket options */
 #define UDP_CORK	1	/* Never send partially complete segments */
 #define UDP_ENCAP	100	/* Set the socket to accept encapsulated packets */
+#define UDP_GRO		104	/* This socket can receive UDP GRO packets */
 
 /* UDP encapsulation types */
 #define UDP_ENCAP_ESPINUDP_NON_IKE	1 /* draft-ietf-ipsec-nat-t-ike-00/01 */