workqueue: cancel_delayed_work() should return %false if work item is idle

57b30ae7 ("workqueue: reimplement cancel_delayed_work() using
try_to_grab_pending()") made cancel_delayed_work() always return %true
unless someone else is also trying to cancel the work item, which is
broken - if the target work item is idle, the return value should be
%false.

try_to_grab_pending() indicates that the target work item was idle by
zero return value.  Use it for return.  Note that this brings
cancel_delayed_work() in line with __cancel_work_timer() in return
value handling.
Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
LKML-Reference: <444a6439-b1a4-4740-9e7e-bc37267cfe73@default>

kernel/workqueue.c

@@ -2982,7 +2982,7 @@ bool cancel_delayed_work(struct delayed_work *dwork)
 
 	set_work_cpu_and_clear_pending(&dwork->work, work_cpu(&dwork->work));
 	local_irq_restore(flags);
-	return true;
+	return ret;
 }
 EXPORT_SYMBOL(cancel_delayed_work);