s390/pci: fix removal of nonexistent pci bus

If we remove a pci bus after receiving a hotplug notification we need
to check if the bus is actually present (creation of the pci bus
during an earlier notification may have been failed).
Reviewed-by: NGerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: NSebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/pci/pci_event.c

@@ -112,6 +112,8 @@ static void __zpci_event_availability(struct zpci_ccdf_avail *ccdf)
 		clp_rescan_pci_devices();
 		break;
 	case 0x0308: /* Standby -> Reserved */
+		if (!zdev)
+			break;
 		pci_stop_root_bus(zdev->bus);
 		pci_remove_root_bus(zdev->bus);
 		break;