powerpc/PCI: Fix NULL dereference in sys_pciconfig_iobase() list traversal

3bc955987fb3 ("powerpc/PCI: Use list_for_each_entry() for bus traversal")
caused a NULL pointer dereference because the loop body set the iterator to
NULL:

  Unable to handle kernel paging request for data at address 0x00000000
  Faulting instruction address: 0xc000000000041d78
  Oops: Kernel access of bad area, sig: 11 [#1]
  ...
  NIP [c000000000041d78] .sys_pciconfig_iobase+0x68/0x1f0
  LR [c000000000041e0c] .sys_pciconfig_iobase+0xfc/0x1f0
  Call Trace:
  [c0000003b4787db0] [c000000000041e0c] .sys_pciconfig_iobase+0xfc/0x1f0 (unreliable)
  [c0000003b4787e30] [c000000000009ed8] syscall_exit+0x0/0x98

Fix it by using a temporary variable for the iterator.

[bhelgaas: changelog, drop tmp_bus initialization]
Fixes: 3bc955987fb3 powerpc/PCI: Use list_for_each_entry() for bus traversal
Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

arch/powerpc/kernel/pci_64.c

@@ -208,7 +208,7 @@ long sys_pciconfig_iobase(long which, unsigned long in_bus,
 			  unsigned long in_devfn)
 {
 	struct pci_controller* hose;
-	struct pci_bus *bus = NULL;
+	struct pci_bus *tmp_bus, *bus = NULL;
 	struct device_node *hose_node;
 
 	/* Argh ! Please forgive me for that hack, but that's the
@@ -229,10 +229,12 @@ long sys_pciconfig_iobase(long which, unsigned long in_bus,
 	 * used on pre-domains setup. We return the first match
 	 */
 
-	list_for_each_entry(bus, &pci_root_buses, node) {
-		if (in_bus >= bus->number && in_bus <= bus->busn_res.end)
+	list_for_each_entry(tmp_bus, &pci_root_buses, node) {
+		if (in_bus >= tmp_bus->number &&
+		    in_bus <= tmp_bus->busn_res.end) {
+			bus = tmp_bus;
 			break;
-		bus = NULL;
+		}
 	}
 	if (bus == NULL || bus->dev.of_node == NULL)
 		return -ENODEV;