workqueue: Move kthread_flush_worker() out of alloc_and_link_pwqs()

kthread_flush_worker() can't be called with wq_pool_mutex held.

Prepare for moving wq_pool_mutex and cpu hotplug lock out of
alloc_and_link_pwqs().

Cc: Zqiang <qiang.zhang1211@gmail.com>
Link: https://lore.kernel.org/lkml/20230920060704.24981-1-qiang.zhang1211@gmail.com/


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