[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/ethernet/eth.c b/net/ethernet/eth.c
index 01ecbe4..0ac2524 100644
--- a/net/ethernet/eth.c
+++ b/net/ethernet/eth.c
@@ -157,7 +157,7 @@
 	unsigned char *rawp;
 
 	skb->dev = dev;
-	skb->mac.raw = skb->data;
+	skb_reset_mac_header(skb);
 	skb_pull(skb, ETH_HLEN);
 	eth = eth_hdr(skb);