[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/ppp_generic.c b/drivers/net/ppp_generic.c
index ef58e41..18f1790 100644
--- a/drivers/net/ppp_generic.c
+++ b/drivers/net/ppp_generic.c
@@ -1685,7 +1685,7 @@
 			skb_pull_rcsum(skb, 2);
 			skb->dev = ppp->dev;
 			skb->protocol = htons(npindex_to_ethertype[npi]);
-			skb->mac.raw = skb->data;
+			skb_reset_mac_header(skb);
 			netif_rx(skb);
 			ppp->dev->last_rx = jiffies;
 		}