[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/arcnet/rfc1051.c b/drivers/net/arcnet/rfc1051.c
index 6d6c69f..2de8877 100644
--- a/drivers/net/arcnet/rfc1051.c
+++ b/drivers/net/arcnet/rfc1051.c
@@ -94,7 +94,7 @@
 	int hdr_size = ARC_HDR_SIZE + RFC1051_HDR_SIZE;
 
 	/* Pull off the arcnet header. */
-	skb->mac.raw = skb->data;
+	skb_reset_mac_header(skb);
 	skb_pull(skb, hdr_size);
 
 	if (pkt->hard.dest == 0)