[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/core/netpoll.c b/net/core/netpoll.c
index 32a9f80..0ad3896 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -324,7 +324,7 @@
 	iph->check    = ip_fast_csum((unsigned char *)iph, iph->ihl);
 
 	eth = (struct ethhdr *) skb_push(skb, ETH_HLEN);
-	skb->mac.raw = skb->data;
+	skb_reset_mac_header(skb);
 	skb->protocol = eth->h_proto = htons(ETH_P_IP);
 	memcpy(eth->h_source, np->local_mac, 6);
 	memcpy(eth->h_dest, np->remote_mac, 6);