workqueue: Make wq_adjust_max_active() round-robin pwqs while activating

wq_adjust_max_active() needs to activate work items after max_active is
increased. Previously, it did that by visiting each pwq once activating all
that could be activated. While this makes sense with per-pwq nr_active,
nr_active will be shared across multiple pwqs for unbound wqs. Then, we'd
want to round-robin through pwqs to be fairer.

In preparation, this patch makes wq_adjust_max_active() round-robin pwqs
while activating. While the activation ordering changes, this shouldn't
cause user-noticeable behavior changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>