mac80211: delete AddBA response timer

We never delete the addBA response timer, which
is typically fine, but if the station it belongs
to is deleted very quickly after starting the BA
session, before the peer had a chance to reply,
the timer may fire after the station struct has
been freed already. Therefore, we need to delete
the timer in a suitable spot -- best when the
session is being stopped (which will happen even
then) in which case the delete will be a no-op
most of the time.

I've reproduced the scenario and tested the fix.

This fixes the crash reported at
http://mid.gmane.org/4CAB6F96.6090701@candelatech.com

Cc: stable@kernel.org
Reported-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/agg-tx.c

@@ -175,6 +175,8 @@ int ___ieee80211_stop_tx_ba_session(struct sta_info *sta, u16 tid,
 
 	set_bit(HT_AGG_STATE_STOPPING, &tid_tx->state);
 
+	del_timer_sync(&tid_tx->addba_resp_timer);
+
 	/*
 	 * After this packets are no longer handed right through
 	 * to the driver but are put onto tid_tx->pending instead,