workqueue: Reimplement ordered workqueue using shared nr_active

Because nr_active used to be tied to pwq, an ordered workqueue had to have a
single pwq to guarantee strict ordering. This led to several contortions to
avoid creating multiple pwqs.

Now that nr_active can be shared across multiple pwqs, we can simplify
ordered workqueue implementation. All that's necessary is ensuring that a
single wq_node_nr_active is shared across all pwqs, which is achieved by
making wq_node_nr_active() always return wq->node_nr_active[nr_node_ids] for
ordered workqueues.

The new implementation is simpler and allows ordered workqueues to share
locality aware worker_pools with other unbound workqueues which should
improve execution locality.

Signed-off-by: Tejun Heo <tj@kernel.org>