[SK_BUFF]: Introduce skb_network_header()

For the places where we need a pointer to the network header, it is still legal
to touch skb->nh.raw directly if just adding to, subtracting from or setting it
to another layer header.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/xfrm4_input.c b/net/ipv4/xfrm4_input.c
index 78e80de..d89969c 100644
--- a/net/ipv4/xfrm4_input.c
+++ b/net/ipv4/xfrm4_input.c
@@ -146,7 +146,7 @@
 		return 0;
 	} else {
 #ifdef CONFIG_NETFILTER
-		__skb_push(skb, skb->data - skb->nh.raw);
+		__skb_push(skb, skb->data - skb_network_header(skb));
 		skb->nh.iph->tot_len = htons(skb->len);
 		ip_send_check(skb->nh.iph);