ACPI / hotplug / PCI: Fix NULL pointer dereference in cleanup_bridge()

After commit bbd34fcd (ACPI / hotplug / PCI: Register all devices
under the given bridge) register_slot() is called for all PCI
devices under a given bridge that have corresponding objects in
the ACPI namespace, but it calls acpiphp_register_hotplug_slot()
only for devices satisfying specific criteria.  Still,
cleanup_bridge() calls acpiphp_unregister_hotplug_slot() for all
objects created by register_slot(), although it should only call it
for the ones that acpiphp_register_hotplug_slot() has been called
for (successfully).  This causes a NULL pointer to be dereferenced
by the acpiphp_unregister_hotplug_slot() executed by cleanup_bridge()
if the object it is called for has not been passed to
acpiphp_register_hotplug_slot().

To fix this problem, check if the 'slot' field of the object passed
to acpiphp_unregister_hotplug_slot() in cleanup_bridge() is not NULL,
which only is the case if acpiphp_register_hotplug_slot() has been
executed for that object.  In addition to that, make register_slot()
reset the 'slot' field to NULL if acpiphp_register_hotplug_slot() has
failed for the given object to prevent stale pointers from being
used by acpiphp_unregister_hotplug_slot().
Reported-and-tested-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/pci/hotplug/acpiphp_glue.c

@@ -340,6 +340,7 @@ static acpi_status register_slot(acpi_handle handle, u32 lvl, void *data,
 
 		retval = acpiphp_register_hotplug_slot(slot, sun);
 		if (retval) {
+			slot->slot = NULL;
 			bridge->nr_slots--;
 			if (retval == -EBUSY)
 				warn("Slot %llu already registered by another "
@@ -429,7 +430,8 @@ static void cleanup_bridge(struct acpiphp_bridge *bridge)
 					err("failed to remove notify handler\n");
 			}
 		}
-		acpiphp_unregister_hotplug_slot(slot);
+		if (slot->slot)
+			acpiphp_unregister_hotplug_slot(slot);
 	}
 
 	mutex_lock(&bridge_mutex);