mac80211: fix another key non-race

The code here is only not racy because all the
places that assign the pointers it uses are
holding the sta_mtx as well as the key_mtx and
so can't race against this because this code
holds the sta_mtx. But that's not intuitive,
so fix it to hold the key_mtx.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/sta_info.c

@@ -652,10 +652,12 @@ static int __must_check __sta_info_destroy(struct sta_info *sta)
 	if (ret)
 		return ret;
 
+	mutex_lock(&local->key_mtx);
 	for (i = 0; i < NUM_DEFAULT_KEYS; i++)
-		ieee80211_key_free(local, sta->gtk[i]);
+		__ieee80211_key_free(sta->gtk[i]);
 	if (sta->ptk)
-		ieee80211_key_free(local, sta->ptk);
+		__ieee80211_key_free(sta->ptk);
+	mutex_unlock(&local->key_mtx);
 
 	sta->dead = true;