[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>

net/llc/llc_output.c

@@ -52,7 +52,7 @@ int llc_mac_hdr_init(struct sk_buff *skb,
 		if (da) {
 			memcpy(trh->daddr, da, dev->addr_len);
 			tr_source_route(skb, trh, dev);
-			skb->mac.raw = skb->data;
+			skb_reset_mac_header(skb);
 		}
 		break;
 	}