[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/ax25/ax25_in.c b/net/ax25/ax25_in.c
index 4a6b26b..6d11b06 100644
--- a/net/ax25/ax25_in.c
+++ b/net/ax25/ax25_in.c
@@ -122,7 +122,7 @@
 		}
 
 		skb_pull(skb, 1);	/* Remove PID */
-		skb->mac.raw  = skb->nh.raw;
+		skb_reset_mac_header(skb);
 		skb->nh.raw   = skb->data;
 		skb->dev      = ax25->ax25_dev->dev;
 		skb->pkt_type = PACKET_HOST;