[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/af_inet.c b/net/ipv4/af_inet.c
index 6e5575b..ab552a6 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1184,7 +1184,7 @@
 		iph->id = htons(id++);
 		iph->tot_len = htons(skb->len - skb->mac_len);
 		iph->check = 0;
-		iph->check = ip_fast_csum(skb->nh.raw, iph->ihl);
+		iph->check = ip_fast_csum(skb_network_header(skb), iph->ihl);
 	} while ((skb = skb->next));
 
 out: