[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/include/net/ax25.h b/include/net/ax25.h
index 47ff2f4..99a4e36 100644
--- a/include/net/ax25.h
+++ b/include/net/ax25.h
@@ -263,8 +263,8 @@
 static inline __be16 ax25_type_trans(struct sk_buff *skb, struct net_device *dev)
 {
 	skb->dev      = dev;
+	skb_reset_mac_header(skb);
 	skb->pkt_type = PACKET_HOST;
-	skb->mac.raw  = skb->data;
 	return htons(ETH_P_AX25);
 }