[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/drivers/net/loopback.c b/drivers/net/loopback.c
index 4380e5e..a71d8e0 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -90,7 +90,7 @@
 		if (!nskb)
 			break;
 		skb_reserve(nskb, 32);
-		nskb->mac.raw = nskb->data - 14;
+		skb_set_mac_header(nskb, -ETH_HLEN);
 		nskb->nh.raw = nskb->data;
 		iph = nskb->nh.iph;
 		memcpy(nskb->data, skb->nh.raw, doffset);