[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/pc300_tty.c

@@ -1003,7 +1003,7 @@ static void cpc_tty_trace(pc300dev_t *dev, char* buf, int len, char rxtx)
 	skb_put (skb, 10 + len); 
 	skb->dev = dev->dev; 
 	skb->protocol = htons(ETH_P_CUST); 
-	skb->mac.raw = skb->data; 
+	skb_reset_mac_header(skb);
 	skb->pkt_type = PACKET_HOST; 
 	skb->len = 10 + len;