workqueue: rename first_worker() to first_idle_worker()

first_worker() actually returns the first idle workers, the name
first_idle_worker() which is self-commnet will be better.

All the callers of first_worker() expect it returns an idle worker,
the name first_idle_worker() with "idle" notation makes reviewers happier.

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