ipv4/ip_gre: set transport header correctly to gre header

ipgre_tunnel_xmit() incorrectly sets transport header to inner payload
instead of GRE header. It seems copy-and-pasted from ipip.c.
So set transport header to gre header.
(In ipip case the transport header is the inner ip header, so that's
correct.)

Found by inspection. In practice the incorrect transport header
doesn't matter because the skb usually is sent to another net_device
or socket, so the transport header isn't referenced.
Signed-off-by: NIsaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -912,9 +912,9 @@ static netdev_tx_t ipgre_tunnel_xmit(struct sk_buff *skb, struct net_device *dev
 		/* Warning : tiph value might point to freed memory */
 	}
 
-	skb_reset_transport_header(skb);
 	skb_push(skb, gre_hlen);
 	skb_reset_network_header(skb);
+	skb_set_transport_header(skb, sizeof(*iph));
 	memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));
 	IPCB(skb)->flags &= ~(IPSKB_XFRM_TUNNEL_SIZE | IPSKB_XFRM_TRANSFORMED |
 			      IPSKB_REROUTED);