[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/drivers/net/irda/stir4200.c b/drivers/net/irda/stir4200.c
index 20d306f..a22175f 100644
--- a/drivers/net/irda/stir4200.c
+++ b/drivers/net/irda/stir4200.c
@@ -364,7 +364,7 @@
 
 	skb_put(skb, len);
 
-	skb->mac.raw  = skb->data;
+	skb_reset_mac_header(skb);
 	skb->protocol = htons(ETH_P_IRDA);
 	skb->dev = stir->netdev;