gro: Fix COMPLETE checksum handling

On a brand new GRO skb, we cannot call ip_hdr since the header
may lie in the non-linear area.  This patch adds the helper
skb_gro_network_header to handle this.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 4b5aa18..d9dd94b 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -943,7 +943,7 @@
 
 struct sk_buff **tcp6_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 {
-	struct ipv6hdr *iph = ipv6_hdr(skb);
+	struct ipv6hdr *iph = skb_gro_network_header(skb);
 
 	switch (skb->ip_summed) {
 	case CHECKSUM_COMPLETE: