[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>

drivers/net/wan/cycx_x25.c

@@ -834,7 +834,7 @@ static void cycx_x25_irq_rx(struct cycx_device *card, struct cycx_x25_cmd *cmd)
 	++chan->ifstats.rx_packets;
 	chan->ifstats.rx_bytes += pktlen;
 
-	skb->mac.raw = skb->data;
+	skb_reset_mac_header(skb);
 	netif_rx(skb);
 	dev->last_rx = jiffies;		/* timestamp */
 }