libertas: Add spinlock to avoid race condition

lbs_mac_event_disconnected may free priv->currenttxskb
while lbs_hard_start_xmit accesses to it.
The patch adds a spinlock for mutual exclusion.

Tested on OLPC XO-1 (usb8388) and XO-1.5 (sd8686) with v4.7-rc3.

Confirmed that lbs_mac_event_disconnected is being called on the
station when hostapd on access point is given SIGHUP.
Signed-off-by: Pavel <andrianov@ispras.ru>
Tested-by: James Cameron <quozl@laptop.org>
Acked-by: Vaishali Thakkar <vaishali.thakkar@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/marvell/libertas/cmdresp.c

@@ -27,6 +27,8 @@
 void lbs_mac_event_disconnected(struct lbs_private *priv,
 				bool locally_generated)
 {
+	unsigned long flags;
+
 	if (priv->connect_status != LBS_CONNECTED)
 		return;
 
@@ -46,9 +48,11 @@ void lbs_mac_event_disconnected(struct lbs_private *priv,
 	netif_carrier_off(priv->dev);
 
 	/* Free Tx and Rx packets */
+	spin_lock_irqsave(&priv->driver_lock, flags);
 	kfree_skb(priv->currenttxskb);
 	priv->currenttxskb = NULL;
 	priv->tx_pending_len = 0;
+	spin_unlock_irqrestore(&priv->driver_lock, flags);
 
 	priv->connect_status = LBS_DISCONNECTED;