[SK_BUFF]: Introduce skb_reset_mac_header(skb)

For the common, open coded 'skb->mac.raw = skb->data' operation, so that we can
later turn skb->mac.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/wanrouter/wanmain.c b/net/wanrouter/wanmain.c
index 5d2d93d..c49e223 100644
--- a/net/wanrouter/wanmain.c
+++ b/net/wanrouter/wanmain.c
@@ -339,7 +339,7 @@
 	skb->protocol = ethertype;
 	skb->pkt_type = PACKET_HOST;	/*	Physically point to point */
 	skb_pull(skb, cnt);
-	skb->mac.raw  = skb->data;
+	skb_reset_mac_header(skb);
 	return ethertype;
 }