[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/net/802/hippi.c b/net/802/hippi.c
index 138302c..d871900 100644
--- a/net/802/hippi.c
+++ b/net/802/hippi.c
@@ -131,7 +131,7 @@
 	 * set the raw address here.
 	 */
 	skb->dev = dev;
-	skb->mac.raw = skb->data;
+	skb_reset_mac_header(skb);
 	hip = (struct hippi_hdr *)skb->mac.raw;
 	skb_pull(skb, HIPPI_HLEN);