workqueue: fix zero @delay handling of queue_delayed_work_on()

If @delay is zero and the dealyed_work is idle, queue_delayed_work()
queues it for immediate execution; however, queue_delayed_work_on()
lacks this logic and always goes through timer regardless of @delay.

This patch moves 0 @delay handling logic from queue_delayed_work() to
queue_delayed_work_on() so that both functions behave the same.

Signed-off-by: Tejun Heo <tj@kernel.org>