[GRE]: Fix hardware checksum modification

The skb_postpull_rcsum introduced a bug to the checksum modification.
Although the length pulled is offset bytes, the origin of the pulling
is the GRE header, not the IP header.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -618,7 +618,7 @@ static int ipgre_rcv(struct sk_buff *skb)
 
 		skb->mac.raw = skb->nh.raw;
 		skb->nh.raw = __pskb_pull(skb, offset);
-		skb_postpull_rcsum(skb, skb->mac.raw, offset);
+		skb_postpull_rcsum(skb, skb->h.raw, offset);
 		memset(&(IPCB(skb)->opt), 0, sizeof(struct ip_options));
 		skb->pkt_type = PACKET_HOST;
 #ifdef CONFIG_NET_IPGRE_BROADCAST