iptunnel: specify protocol outside IP header

Before this patch, ip_tunnel_xmit() was using the field protocol from the IP
header passed into argument.
There is no functional change, this patch prepares the support of IPv4 over
IPv4 for module sit.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.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 2a83591..a982657 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -429,7 +429,7 @@
 		return;
 	}
 
-	ip_tunnel_xmit(skb, dev, tnl_params);
+	ip_tunnel_xmit(skb, dev, tnl_params, tnl_params->protocol);
 }
 
 static netdev_tx_t ipgre_xmit(struct sk_buff *skb,