mac80211: fix initialization of skb->cb in ieee80211_subif_start_xmit

The change 'mac80211: Fix BUG in pskb_expand_head when transmitting shared skbs'
added a check for copying the skb if it's shared, however the tx info variable
still points at the cb of the old skb

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/tx.c

@@ -1739,7 +1739,7 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
 {
 	struct ieee80211_sub_if_data *sdata = IEEE80211_DEV_TO_SUB_IF(dev);
 	struct ieee80211_local *local = sdata->local;
-	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
+	struct ieee80211_tx_info *info;
 	int ret = NETDEV_TX_BUSY, head_need;
 	u16 ethertype, hdrlen,  meshhdrlen = 0;
 	__le16 fc;
@@ -2028,6 +2028,7 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
 	skb_set_network_header(skb, nh_pos);
 	skb_set_transport_header(skb, h_pos);
 
+	info = IEEE80211_SKB_CB(skb);
 	memset(info, 0, sizeof(*info));
 
 	dev->trans_start = jiffies;