mac80211: also expire filtered frames

mac80211 will expire normal PS-buffered frames, but
if the device rejected some frames for a sleeping
station, these won't be on the ps_tx_buf queue but
on the tx_filtered queue instead; this is done to
avoid reordering.

However, mac80211 will not expire frames from the
filtered queue, let's fix that.

Also add a more comments to what all this expiry is
doing and how it works.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/net/mac80211/status.c b/net/mac80211/status.c
index 8354dcb..783542a 100644
--- a/net/mac80211/status.c
+++ b/net/mac80211/status.c
@@ -107,6 +107,11 @@
 	    skb_queue_len(&sta->tx_filtered) < STA_MAX_TX_BUFFER) {
 		skb_queue_tail(&sta->tx_filtered, skb);
 		sta_info_recalc_tim(sta);
+
+		if (!timer_pending(&local->sta_cleanup))
+			mod_timer(&local->sta_cleanup,
+				  round_jiffies(jiffies +
+						STA_INFO_CLEANUP_INTERVAL));
 		return;
 	}