gre: Set inner protocol in v4 and v6 GRE transmit

Call skb_set_inner_protocol to set inner Ethernet protocol to
protocol being encapsulation by GRE before tunnel_xmit. This is
needed for GSO if UDP encapsulation (fou) is being done.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 829aff8b..0485ef1 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -241,6 +241,8 @@
 	/* Push GRE header. */
 	gre_build_header(skb, &tpi, tunnel->tun_hlen);
 
+	skb_set_inner_protocol(skb, tpi.proto);
+
 	ip_tunnel_xmit(skb, dev, tnl_params, tnl_params->protocol);
 }