[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/include/net/inet_ecn.h b/include/net/inet_ecn.h
index 10117c8..b9ed389 100644
--- a/include/net/inet_ecn.h
+++ b/include/net/inet_ecn.h
@@ -114,12 +114,14 @@
 {
 	switch (skb->protocol) {
 	case __constant_htons(ETH_P_IP):
-		if (skb->nh.raw + sizeof(struct iphdr) <= skb->tail)
+		if (skb_network_header(skb) + sizeof(struct iphdr) <=
+		    skb->tail)
 			return IP_ECN_set_ce(skb->nh.iph);
 		break;
 
 	case __constant_htons(ETH_P_IPV6):
-		if (skb->nh.raw + sizeof(struct ipv6hdr) <= skb->tail)
+		if (skb_network_header(skb) + sizeof(struct ipv6hdr) <=
+		    skb->tail)
 			return IP6_ECN_set_ce(skb->nh.ipv6h);
 		break;
 	}