gtp: reset mac_header after decap

For consistency with other L3 tunnel devices, reset the mac_header
pointer after decapsulation. This makes the mac_header 0 bytes long,
thus making it clear that this skb has no mac_header.

Compile tested only.
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/gtp.c

@@ -201,6 +201,7 @@ static int gtp_rx(struct pdp_ctx *pctx, struct sk_buff *skb,
 	 * calculate the transport header.
 	 */
 	skb_reset_network_header(skb);
+	skb_reset_mac_header(skb);
 
 	skb->dev = pctx->dev;