[SK_BUFF]: Introduce skb_reset_network_header(skb)

For the common, open coded 'skb->nh.raw = skb->data' operation, so that we can
later turn skb->nh.raw into a offset, reducing the size of struct sk_buff in
64bit land while possibly keeping it as a pointer on 32bit.

This one touches just the most simple case, next will handle the slightly more
"complex" cases.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index e6e196c..8c533ce 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -578,7 +578,7 @@
 		return NULL;
 
 	skb_reserve(skb, LL_RESERVED_SPACE(dev));
-	skb->nh.raw = skb->data;
+	skb_reset_network_header(skb);
 	arp = (struct arphdr *) skb_put(skb,sizeof(struct arphdr) + 2*(dev->addr_len+4));
 	skb->dev = dev;
 	skb->protocol = htons(ETH_P_ARP);