workqueue: make cancel_rearming_delayed_workqueue() work on idle dwork

cancel_rearming_delayed_workqueue(dwork) will hang forever if dwork was not
scheduled, because in that case cancel_delayed_work()->del_timer_sync() never
returns true.

I don't know if there are any callers which may have problems, but this is not
so convenient, and the fix is very simple.

Q: looks like we don't need "struct workqueue_struct *wq" parameter.  If the
timer was aborted successfully, get_wq_data() == wq.  Is it worth to add the
new function?

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/workqueue.c

@@ -569,6 +569,10 @@ EXPORT_SYMBOL(flush_work_keventd);
 void cancel_rearming_delayed_workqueue(struct workqueue_struct *wq,
 				       struct delayed_work *dwork)
 {
+	/* Was it ever queued ? */
+	if (!get_wq_data(&dwork->work))
+		return;
+
 	while (!cancel_delayed_work(dwork))
 		flush_workqueue(wq);
 }