workqueue: alloc struct worker on its local node

When the create_worker() is called from non-manager, the struct worker
is allocated from the node of the caller which may be different from the
node of pool->node.

So we add a node ID argument for the alloc_worker() to ensure the
struct worker is allocated from the preferable node.

tj: @nid renamed to @node for consistency.

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