PCI/ACPI: fix wrong assumption in acpi_pci_get_bridge_handle

PCI/ACPI: fix wrong assumption in acpi_pci_get_bridge_handle

Current acpi_pci_get_bridge_handle() has an assumption that
pci_bus->self is NULL on the root pci bus. But it might not true on
some platforms. Because of this wrong assumption, current
acpi_pci_get_bridge_handle() might return improper ACPI handle. We
must check pci_bus->parent instead.

This bug is the root cause of the following kernel panic reported by
James Bottomley. This problem was introduced by the commit
e8c331e963c58b83db24b7d0e39e8c07f687dbc6. The immediate cause was
acpi_pci_get_bridge_handle() returned NULL unexpectedly and it was
passed as the second argument of acpi_walk_namespace().

pci_hotplug: PCI Hot Plug PCI Core version: 0.5
acpiphp: ACPI Hot Plug PCI Controller Driver version: 0.5
BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
IP: [<ffffffff8039646f>] acpi_ns_get_next_node+0xb/0x3c
PGD 0
Oops: 0000 [#1] SMP
last sysfs file:
CPU 0
Modules linked in:
Pid: 1, comm: swapper Not tainted 2.6.28 #1
RIP: 0010:[<ffffffff8039646f>]  [<ffffffff8039646f>] acpi_ns_get_next_node+0xb/0x3c
RSP: 0018:ffff88007f87fd30  EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffffffff8037d260 R09: ffff88007f87fdfc
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000001
R13: 0000000000000000 R14: 0000000000000001 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffffffff80742040(0000) knlGS:0000000000000000
CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
CR2: 0000000000000010 CR3: 0000000000201000 CR4: 00000000000006a0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process swapper (pid: 1, threadinfo ffff88007f87e000, task ffff88007f875040)
Stack:
 0000000000000000 ffffffff803964f5 ffff88007f81b728 0000000000001001
 ffff88007f87fdfc ffffffff8037d260 0000000600000001 0000000000000000
 ffffffff8037d260 0000000000000000 0000000000000001 ffff88007f87fdfc
Call Trace:
 [<ffffffff803964f5>] acpi_ns_walk_namespace+0x55/0x138
 [<ffffffff8037d260>] is_pci_dock_device+0x0/0x20
 [<ffffffff8037d260>] is_pci_dock_device+0x0/0x20
 [<ffffffff80394a9e>] acpi_walk_namespace+0x5f/0x83
 [<ffffffff8037dd33>] detect_ejectable_slots+0x53/0x70
 [<ffffffff8037de38>] add_bridge+0xe8/0x200
 [<ffffffff80394aaa>] acpi_walk_namespace+0x6b/0x83
 [<ffffffff803a4ad1>] acpi_pci_register_driver+0x48/0x61
 [<ffffffff806fc5df>] acpiphp_init+0x0/0x58
 [<ffffffff806fc732>] acpiphp_glue_init+0x4c/0x5a
 [<ffffffff806fc616>] acpiphp_init+0x37/0x58
 [<ffffffff8020903b>] _stext+0x3b/0x180
 [<ffffffff80312598>] create_proc_entry+0x58/0xa0
 [<ffffffff802815d1>] register_irq_proc+0xc1/0xe0
 [<ffffffff806db64b>] kernel_init+0x152/0x1ac
 [<ffffffff8023d970>] finish_task_switch+0x0/0x110
 [<ffffffff8020ca7a>] child_rip+0xa/0x20
 [<ffffffff8020c47c>] restore_args+0x0/0x30
 [<ffffffff806db4f9>] kernel_init+0x0/0x1ac
 [<ffffffff8020ca70>] child_rip+0x0/0x20
Code: 89 c2 48 8b 00 48 85 c0 75 f5 48 8b 45 00 48 89 02 44 88 65 09 48 89 5d 00 31 c0 5b 5d 41 5c c3 53 48 85 d2 89 fb 48 89 d7 75 06 <48> 8b 56 10 eb 08 e8 73 f1 ff ff 48 89 c2 85 db 74 1a eb 13 0f
RIP  [<ffffffff8039646f>] acpi_ns_get_next_node+0xb/0x3c
 RSP <ffff88007f87fd30>
CR2: 0000000000000010
---[ end trace a7919e7f17c0a725 ]---

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>