PCI: Flush PCI probe workqueue on cpuset isolated partition change

PCI: Flush PCI probe workqueue on cpuset isolated partition change

The HK_TYPE_DOMAIN housekeeping cpumask is now modifiable at runtime. In
order to synchronize against PCI probe works and make sure that no
asynchronous probing is still pending or executing on a newly isolated
CPU, the housekeeping subsystem must flush the PCI probe works.

However the PCI probe works can't be flushed easily since they are
queued to the main per-CPU workqueue pool.

Solve this with creating a PCI probe-specific pool and provide and use
the appropriate flushing API.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Marco Crivellari <marco.crivellari@suse.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Waiman Long <longman@redhat.com>
Cc: linux-pci@vger.kernel.org