ipv4: gre: add GRO capability

Add GRO capability to IPv4 GRE tunnels, using the gro_cells
infrastructure.

Tested using IPv4 and IPv6 TCP traffic inside this tunnel, and
checking GRO is building large packets.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/ipip.h b/include/net/ipip.h
index a93cf6d..ddc077c 100644
--- a/include/net/ipip.h
+++ b/include/net/ipip.h
@@ -2,6 +2,7 @@
 #define __NET_IPIP_H 1
 
 #include <linux/if_tunnel.h>
+#include <net/gro_cells.h>
 #include <net/ip.h>
 
 /* Keep error state on tunnel for 30 sec */
@@ -36,6 +37,8 @@
 #endif
 	struct ip_tunnel_prl_entry __rcu *prl;		/* potential router list */
 	unsigned int			prl_count;	/* # of entries in PRL */
+
+	struct gro_cells		gro_cells;
 };
 
 struct ip_tunnel_prl_entry {