mac80211: Fixing sparse warning at sta_info.c

The commit 42624d49
created following sparse warning
>net/mac80211/sta_info.c:965:24: warning: incorrect type in assignment (different address spaces)
>net/mac80211/sta_info.c:965:24:    expected struct tid_ampdu_tx *tid_tx
>net/mac80211/sta_info.c:965:24:    got struct tid_ampdu_tx [noderef] <asn:4>*<noident>

Making use of rcu_dereference_protected to fix the problem.

V2:
 - Replacing rcu_dereference with rcu_dereference_protected
   as suggested by Johannes.
 - Adding mutex_lock/unlock to satisfy the condition at
   rcu_dereference_protected

Cc: Nishant Sarmukadam <nishants@marvell.com>
Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/sta_info.c

@@ -822,10 +822,13 @@ static int __must_check __sta_info_destroy(struct sta_info *sta)
 	 * until the aggregation stop completes. Refer
 	 * http://thread.gmane.org/gmane.linux.kernel.wireless.general/81936
 	 */
+
+	mutex_lock(&sta->ampdu_mlme.mtx);
+
 	for (i = 0; i < STA_TID_NUM; i++) {
-		if (!sta->ampdu_mlme.tid_tx[i])
+		tid_tx = rcu_dereference_protected_tid_tx(sta, i);
+		if (!tid_tx)
 			continue;
-		tid_tx = sta->ampdu_mlme.tid_tx[i];
 		if (skb_queue_len(&tid_tx->pending)) {
 #ifdef CONFIG_MAC80211_HT_DEBUG
 			wiphy_debug(local->hw.wiphy, "TX A-MPDU  purging %d "
@@ -837,6 +840,8 @@ static int __must_check __sta_info_destroy(struct sta_info *sta)
 		kfree_rcu(tid_tx, rcu_head);
 	}
 
+	mutex_unlock(&sta->ampdu_mlme.mtx);
+
 	sta_info_free(local, sta);
 
 	return 0;