PCI: pci_slot: grab refcount on slot's bus

If a logical hot unplug (remove) is performed on a physical PCI slot's
parent bridge, and then pci_slot is unloaded, we will encounter an oops:

[<ffffffff803a788a>] kobject_release+0x9a/0x290
[<ffffffff803a77f0>] ? kobject_release+0x0/0x290
[<ffffffff803a8ce7>] kref_put+0x37/0x80
[<ffffffff803a76f7>] kobject_put+0x27/0x60
[<ffffffff803bebcc>] ? pci_destroy_slot+0x3c/0xc0
[<ffffffff803bebd5>] pci_destroy_slot+0x45/0xc0
[<ffffffffa000f05c>] acpi_pci_slot_remove+0x5c/0x91 [pci_slot]
[<ffffffff8040064b>] acpi_pci_unregister_driver+0x4b/0x62
[<ffffffffa000f5c8>] acpi_pci_slot_exit+0x10/0x12 [pci_slot]
[<ffffffff80276ce1>] sys_delete_module+0x161/0x250

We need to grab a reference to the parent PCI bus, which will pin
the bus and prevent it from being released until pci_slot is unloaded.

Cc: [e-mail]
Reviewed-by: Kenji Kaneshige <[e-mail]>
Tested-by: Kenji Kaneshige <[e-mail]>
Reported-by: Kenji Kaneshige <[e-mail]>
Signed-off-by: Alex Chiang <[e-mail]>
Signed-off-by: Jesse Barnes <[e-mail]>

drivers/acpi/pci_slot.c

···
 	list_add(&slot->list, &slot_list);
 	mutex_unlock(&slot_list_lock);
 
+	get_device(&pci_bus->dev);
+
 	dbg("pci_slot: %p, pci_bus: %x, device: %d, name: %s\n",
 		pci_slot, pci_bus->number, device, name);
 
···
 acpi_pci_slot_remove(acpi_handle handle)
 {
 	struct acpi_pci_slot *slot, *tmp;
+	struct pci_bus *pbus;
 
 	mutex_lock(&slot_list_lock);
 	list_for_each_entry_safe(slot, tmp, &slot_list, list) {
 		if (slot->root_handle == handle) {
 			list_del(&slot->list);
+			pbus = slot->pci_slot->bus;
 			pci_destroy_slot(slot->pci_slot);
+			put_device(&pbus->dev);
 			kfree(slot);
 		}
 	}