workqueue: Register sysfs after the whole creation of the new wq

workqueue creation includes adding it to the workqueue list.

Prepare for moving the whole workqueue initializing procedure into
wq_pool_mutex and cpu hotplug locks.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Tejun Heo <tj@kernel.org>