[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/ip_fragment.c b/net/ipv4/ip_fragment.c
index 3dfd758..268a6c7 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -658,7 +658,7 @@
 	}
 
 	skb_shinfo(head)->frag_list = head->next;
-	skb_push(head, head->data - head->nh.raw);
+	skb_push(head, head->data - skb_network_header(head));
 	atomic_sub(head->truesize, &ip_frag_mem);
 
 	for (fp=head->next; fp; fp = fp->next) {