PCI: acpiphp: Protect acpiphp data structures from concurrent updates

Now acpiphp_enumerate_slots() and acpiphp_remove_slots() may be invoked
concurrently by the PCI core, so add a bridge_mutex and reference count
mechanism to protect acpiphp bridge/slot/function data structures.

To avoid deadlock, handle_hotplug_event_bridge() will requeue the
hotplug event onto the kacpi_hotplug_wq by calling alloc_acpi_hp_work().
But the workaround has introduced a minor race window because the
'bridge' passed to _handle_hotplug_event_bridge() may have already been
destroyed when _handle_hotplug_event_bridge() is actually executed by
the kacpi_hotplug_wq.  So hold a reference count on the passed 'bridge'.
Fix the same issue for handle_hotplug_event_func() too.

Signed-off-by: Jiang Liu <[email protected]>
Signed-off-by: Yijing Wang <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Reviewed-by: Yinghai Lu <[email protected]>
Cc: "Rafael J. Wysocki" <[email protected]>
Cc: Toshi Kani <[email protected]>

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