iwlwifi: add missing rcu_read_lock

commit 6db6340c upstream.

Using ieee80211_find_sta() needs to be under
RCU read lock, which iwlwifi currently misses,
so fix it.
Reported-by: Miles Lane <miles.lane@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Miles Lane <miles.lane@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/net/wireless/iwlwifi/iwl-tx.c

@@ -1198,6 +1198,7 @@ static void iwl_tx_status(struct iwl_priv *priv, struct sk_buff *skb)
 	struct ieee80211_sta *sta;
 	struct iwl_station_priv *sta_priv;
 
+	rcu_read_lock();
 	sta = ieee80211_find_sta(priv->vif, hdr->addr1);
 	if (sta) {
 		sta_priv = (void *)sta->drv_priv;
@@ -1206,6 +1207,7 @@ static void iwl_tx_status(struct iwl_priv *priv, struct sk_buff *skb)
 		    atomic_dec_return(&sta_priv->pending_frames) == 0)
 			ieee80211_sta_block_awake(priv->hw, sta, false);
 	}
+	rcu_read_unlock();
 
 	ieee80211_tx_status_irqsafe(priv->hw, skb);
 }