[SK_BUFF]: Introduce skb_set_mac_header()

For the cases where we want to set skb->mac.raw to an offset from skb->data.

Simple cases first, the memmove ones and specially pktgen will be left for later.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/atm/br2684.c b/net/atm/br2684.c
index 900d42c..a1686df 100644
--- a/net/atm/br2684.c
+++ b/net/atm/br2684.c
@@ -458,7 +458,7 @@
 	/* FIXME: tcpdump shows that pointer to mac header is 2 bytes earlier,
 	   than should be. What else should I set? */
 	skb_pull(skb, plen);
-	skb->mac.raw = skb->data - ETH_HLEN;
+	skb_set_mac_header(skb, -ETH_HLEN);
 	skb->pkt_type = PACKET_HOST;
 #ifdef CONFIG_BR2684_FAST_TRANS
 	skb->protocol = ((u16 *) skb->data)[-1];