[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/drivers/net/wan/farsync.c b/drivers/net/wan/farsync.c
index c45d6a8..58a53b6 100644
--- a/drivers/net/wan/farsync.c
+++ b/drivers/net/wan/farsync.c
@@ -864,7 +864,7 @@
 static __be16 farsync_type_trans(struct sk_buff *skb, struct net_device *dev)
 {
 	skb->dev = dev;
-	skb->mac.raw = skb->data;
+	skb_reset_mac_header(skb);
 	skb->pkt_type = PACKET_HOST;
 	return htons(ETH_P_CUST);
 }