[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>

drivers/infiniband/ulp/ipoib/ipoib_cm.c

@@ -408,7 +408,7 @@ void ipoib_cm_handle_rx_wc(struct net_device *dev, struct ib_wc *wc)
 	skb_put_frags(skb, IPOIB_CM_HEAD_SIZE, wc->byte_len, newskb);
 
 	skb->protocol = ((struct ipoib_header *) skb->data)->proto;
-	skb->mac.raw = skb->data;
+	skb_reset_mac_header(skb);
 	skb_pull(skb, IPOIB_ENCAP_LEN);
 
 	dev->last_rx = jiffies;

drivers/infiniband/ulp/ipoib/ipoib_ib.c

@@ -216,7 +216,7 @@ static void ipoib_ib_handle_rx_wc(struct net_device *dev, struct ib_wc *wc)
 	if (wc->slid != priv->local_lid ||
 	    wc->src_qp != priv->qp->qp_num) {
 		skb->protocol = ((struct ipoib_header *) skb->data)->proto;
-		skb->mac.raw = skb->data;
+		skb_reset_mac_header(skb);
 		skb_pull(skb, IPOIB_ENCAP_LEN);
 
 		dev->last_rx = jiffies;