[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/isdn/i4l/isdn_ppp.c b/drivers/isdn/i4l/isdn_ppp.c
index 1b2df80..be91505 100644
--- a/drivers/isdn/i4l/isdn_ppp.c
+++ b/drivers/isdn/i4l/isdn_ppp.c
@@ -1167,7 +1167,7 @@
 		mlp->huptimer = 0;
 #endif /* CONFIG_IPPP_FILTER */
 	skb->dev = dev;
-	skb->mac.raw = skb->data;
+	skb_reset_mac_header(skb);
 	netif_rx(skb);
 	/* net_dev->local->stats.rx_packets++; done in isdn_net.c */
 	return;