ipip: Add gro callbacks to ipip offload

Add inet_gro_receive and inet_gro_complete to ipip_offload to
support GRO.
Signed-off-by: NTom Herbert <therbert@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/af_inet.c

@@ -1666,6 +1666,8 @@ static const struct net_offload ipip_offload = {
 	.callbacks = {
 		.gso_send_check = inet_gso_send_check,
 		.gso_segment	= inet_gso_segment,
+		.gro_receive	= inet_gro_receive,
+		.gro_complete	= inet_gro_complete,
 	},
 };