Currently, the code to attach a new worker to its pool is embedded in
create_worker().  Separating this code out will make the codes clearer
and will allow rescuers to share the code path later.

tj: Description and comment updates.
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: NTejun Heo <tj@kernel.org>