[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/wireless/hostap/hostap_80211_tx.c b/drivers/net/wireless/hostap/hostap_80211_tx.c
index 4a5be70..159baef 100644
--- a/drivers/net/wireless/hostap/hostap_80211_tx.c
+++ b/drivers/net/wireless/hostap/hostap_80211_tx.c
@@ -237,7 +237,7 @@
 	iface->stats.tx_packets++;
 	iface->stats.tx_bytes += skb->len;
 
-	skb->mac.raw = skb->data;
+	skb_reset_mac_header(skb);
 	meta = (struct hostap_skb_tx_data *) skb->cb;
 	memset(meta, 0, sizeof(*meta));
 	meta->magic = HOSTAP_SKB_TX_DATA_MAGIC;