[IPV4] ip_gre: set mac_header correctly in receive path

mac_header update in ipgre_recv() was incorrectly changed to
skb_reset_mac_header() when it was introduced.
Signed-off-by: NTimo Teras <timo.teras@iki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -613,7 +613,7 @@ static int ipgre_rcv(struct sk_buff *skb)
 				offset += 4;
 		}
 
-		skb_reset_mac_header(skb);
+		skb->mac_header = skb->network_header;
 		__pskb_pull(skb, offset);
 		skb_reset_network_header(skb);
 		skb_postpull_rcsum(skb, skb_transport_header(skb), offset);