vxlan: GRO support at tunnel layer

Add calls to gro_cells infrastructure to do GRO when receiving on a tunnel.

Testing:

Ran 200 netperf TCP_STREAM instance

  - With fix (GRO enabled on VXLAN interface)

    Verify GRO is happening.

    9084 MBps tput
    3.44% CPU utilization

  - Without fix (GRO disabled on VXLAN interface)

    Verified no GRO is happening.

    9084 MBps tput
    5.54% CPU utilization

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/vxlan.h b/include/net/vxlan.h
index 43677e6..6b32345 100644
--- a/include/net/vxlan.h
+++ b/include/net/vxlan.h
@@ -161,6 +161,7 @@
 	struct timer_list age_timer;
 	spinlock_t	  hash_lock;
 	unsigned int	  addrcnt;
+	struct gro_cells  gro_cells;
 
 	struct vxlan_config	cfg;