workqueue: use manager lock only to protect worker_idr

worker_idr is highly bound to managers and is always/only accessed in manager
lock context. So we don't need pool->lock for it.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: XileForce <rdaguiar360@gmail.com>

Original commit: https://github.com/ktoonsez/KTSGS6/commit/e95a650ebfaf915ccd702039ff110dd36d7a3f56