[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/irda/nsc-ircc.c

@@ -1881,7 +1881,7 @@ static int nsc_ircc_dma_receive_complete(struct nsc_ircc_cb *self, int iobase)
 			self->stats.rx_packets++;
 
 			skb->dev = self->netdev;
-			skb->mac.raw  = skb->data;
+			skb_reset_mac_header(skb);
 			skb->protocol = htons(ETH_P_IRDA);
 			netif_rx(skb);
 			self->netdev->last_rx = jiffies;