workqueue: trivial fix for return statement in work_busy()

Return type of work_busy() is unsigned int.
There is return statement returning boolean value, 'false' in work_busy().
It is not problem, because 'false' may be treated '0'.
However, fixing it would make code robust.
Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -3485,7 +3485,7 @@ unsigned int work_busy(struct work_struct *work)
 	unsigned int ret = 0;
 
 	if (!gcwq)
-		return false;
+		return 0;
 
 	spin_lock_irqsave(&gcwq->lock, flags);