[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/s390/net/qeth_eddp.c b/drivers/s390/net/qeth_eddp.c
index 7c735e1..910a8ab 100644
--- a/drivers/s390/net/qeth_eddp.c
+++ b/drivers/s390/net/qeth_eddp.c
@@ -486,7 +486,7 @@
 		return -ENOMEM;
 	}
 	if (qhdr->hdr.l2.id == QETH_HEADER_TYPE_LAYER2) {
-		skb->mac.raw = (skb->data) + sizeof(struct qeth_hdr);
+		skb->mac.raw = skb->data + sizeof(struct qeth_hdr);
 		memcpy(&eddp->mac, eth_hdr(skb), ETH_HLEN);
 #ifdef CONFIG_QETH_VLAN
 		if (eddp->mac.h_proto == __constant_htons(ETH_P_8021Q)) {