workqueue: Call wq_update_unbound_numa() on all CPUs in NUMA node on CPU hotplug

When a CPU went online or offline, wq_update_unbound_numa() was called only
on the CPU which was going up or down. This works fine because all CPUs on
the same NUMA node share the same pool_workqueue slot - one CPU updating it
updates it for everyone in the node.

However, future changes will make each CPU use a separate pool_workqueue
even when they're sharing the same worker_pool, which requires updating
pool_workqueue's for all CPUs which may be sharing the same pool_workqueue
on hotplug.

To accommodate the planned changes, this patch updates
workqueue_on/offline_cpu() so that they call wq_update_unbound_numa() for
all CPUs sharing the same NUMA node as the CPU going up or down. In the
current code, the second+ calls would be noops and there shouldn't be any
behavior changes.

* As wq_update_unbound_numa() is now called on multiple CPUs per each
  hotplug event, @cpu is renamed to @hotplug_cpu and another @cpu argument
  is added. The former indicates the CPU being hot[un]plugged and the latter
  the CPU whose pool_workqueue is being updated.

* In wq_update_unbound_numa(), cpu_off is renamed to off_cpu for consistency
  with the new @hotplug_cpu.

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