workqueue: use schedule_timeout_interruptible() instead of open code

schedule_timeout_interruptible(CREATE_COOLDOWN) is exactly the same as
the original code.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Original commit: https://github.com/ktoonsez/KTSGS6/commit/439d0f1a8a16d093703287218395b0f3da1d1739