cfq_exit_queue() should cancel cfq_data->unplug_work

Spotted by Nick <gentuu@gmail.com>, perhaps explains the first trace in
http://bugzilla.kernel.org/show_bug.cgi?id=9180.

cfq_exit_queue() should cancel cfqd->unplug_work before freeing cfqd.
blk_sync_queue() seems unneeded, removed.

Q: why cfq_exit_queue() calls cfq_shutdown_timer_wq() twice?
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/cfq-iosched.c

@@ -2053,7 +2053,7 @@ static void cfq_shutdown_timer_wq(struct cfq_data *cfqd)
 {
 	del_timer_sync(&cfqd->idle_slice_timer);
 	del_timer_sync(&cfqd->idle_class_timer);
-	blk_sync_queue(cfqd->queue);
+	kblockd_flush_work(&cfqd->unplug_work);
 }
 
 static void cfq_put_async_queues(struct cfq_data *cfqd)