[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/appletalk/ltpc.c b/drivers/net/appletalk/ltpc.c
index 2ea44ce..1268243 100644
--- a/drivers/net/appletalk/ltpc.c
+++ b/drivers/net/appletalk/ltpc.c
@@ -770,7 +770,7 @@
 	skb->data[0] = dnode;
 	skb->data[1] = snode;
 	skb->data[2] = llaptype;
-	skb->mac.raw = skb->data;	/* save pointer to llap header */
+	skb_reset_mac_header(skb);	/* save pointer to llap header */
 	skb_pull(skb,3);
 
 	/* copy ddp(s,e)hdr + contents */