[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/decnet/dn_route.c b/net/decnet/dn_route.c
index c1b5502..ef94ca5 100644
--- a/net/decnet/dn_route.c
+++ b/net/decnet/dn_route.c
@@ -1537,7 +1537,7 @@
 	skb = alloc_skb(NLMSG_GOODSIZE, GFP_KERNEL);
 	if (skb == NULL)
 		return -ENOBUFS;
-	skb->mac.raw = skb->data;
+	skb_reset_mac_header(skb);
 	cb = DN_SKB_CB(skb);
 
 	if (rta[RTA_SRC-1])