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>

net/ipv4/tcp_ipv4.c

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 5d427f8..bda74e8 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2343,7 +2343,7 @@
 
 struct sk_buff **tcp4_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 {
-	struct iphdr *iph = ip_hdr(skb);
+	struct iphdr *iph = skb_gro_network_header(skb);
 
 	switch (skb->ip_summed) {
 	case CHECKSUM_COMPLETE: