[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/ah4.c b/net/ipv4/ah4.c
index 7194eb4..95ddbbd 100644
--- a/net/ipv4/ah4.c
+++ b/net/ipv4/ah4.c
@@ -154,7 +154,7 @@
 	ah = (struct ip_auth_hdr*)skb->data;
 	iph = skb->nh.iph;
 
-	ihl = skb->data - skb->nh.raw;
+	ihl = skb->data - skb_network_header(skb);
 	memcpy(work_buf, iph, ihl);
 
 	iph->ttl = 0;
@@ -181,7 +181,8 @@
 		}
 	}
 	((struct iphdr*)work_buf)->protocol = ah->nexthdr;
-	skb->h.raw = memcpy(skb->nh.raw += ah_hlen, work_buf, ihl);
+	skb->nh.raw += ah_hlen;
+	skb->h.raw = memcpy(skb_network_header(skb), work_buf, ihl);
 	__skb_pull(skb, ah_hlen + ihl);
 
 	return 0;