mac80211: cancel restart_work in ieee80211_unregister_hw

Unlike most other workqueue-tasks, the restart_work is
not scheduled onto mac80211's private per-interface
workqueue, but onto one of the system-wide workqueues.

Therefore the mac80211-stack has to cancel any pending
restarts, before destroying the shared device context
and handing back the memory. Otherwise - under very
unlucky circumstances - there could be a stale work-
item left, because some other kernel component might
have delayed the execution of ieee80211_restart_work
for too long.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/main.c

@@ -807,6 +807,7 @@ void ieee80211_unregister_hw(struct ieee80211_hw *hw)
 
 	rtnl_unlock();
 
+	cancel_work_sync(&local->restart_work);
 	cancel_work_sync(&local->reconfig_filter);
 
 	ieee80211_clear_tx_pending(local);