gro: Fix handling of complete checksums in IPv6

We need to perform skb_postpull_rcsum after pulling the IPv6
header in order to maintain the correctness of the complete
checksum.

This patch also adds a missing iph reload after pulling.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/af_inet6.c

@@ -797,6 +797,7 @@ static struct sk_buff **ipv6_gro_receive(struct sk_buff **head,
 	unsigned int nlen;
 	int flush = 1;
 	int proto;
+	__wsum csum;
 
 	if (unlikely(!pskb_may_pull(skb, sizeof(*iph))))
 		goto out;
@@ -808,6 +809,7 @@ static struct sk_buff **ipv6_gro_receive(struct sk_buff **head,
 
 	rcu_read_lock();
 	proto = ipv6_gso_pull_exthdrs(skb, iph->nexthdr);
+	iph = ipv6_hdr(skb);
 	IPV6_GRO_CB(skb)->proto = proto;
 	ops = rcu_dereference(inet6_protos[proto]);
 	if (!ops || !ops->gro_receive)
@@ -839,8 +841,13 @@ static struct sk_buff **ipv6_gro_receive(struct sk_buff **head,
 
 	NAPI_GRO_CB(skb)->flush |= flush;
 
+	csum = skb->csum;
+	skb_postpull_rcsum(skb, iph, skb_network_header_len(skb));
+
 	pp = ops->gro_receive(head, skb);
 
+	skb->csum = csum;
+
 out_unlock:
 	rcu_read_unlock();