driver-core: do not register a driver with bus_type not registered

If the bus_type is not registerd, driver_register to that bus will cause oops.

I found this bug when test built-in usb serial drivers (ie. aircable driver)
with 'nousb' cmdline params.

In this patch:
1. set the bus->p=NULL when bus_register failed and unregisterd.
2. if bus->p is NULL, driver_register BUG_ON will be triggered.

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/base/bus.c

@@ -932,6 +932,7 @@ bus_uevent_fail:
 	kset_unregister(&bus->p->subsys);
 	kfree(bus->p);
 out:
+	bus->p = NULL;
 	return retval;
 }
 EXPORT_SYMBOL_GPL(bus_register);
@@ -953,6 +954,7 @@ void bus_unregister(struct bus_type *bus)
 	bus_remove_file(bus, &bus_attr_uevent);
 	kset_unregister(&bus->p->subsys);
 	kfree(bus->p);
+	bus->p = NULL;
 }
 EXPORT_SYMBOL_GPL(bus_unregister);
 

drivers/base/driver.c

@@ -216,6 +216,8 @@ int driver_register(struct device_driver *drv)
 	int ret;
 	struct device_driver *other;
 
+	BUG_ON(!drv->bus->p);
+
 	if ((drv->bus->probe && drv->probe) ||
 	    (drv->bus->remove && drv->remove) ||
 	    (drv->bus->shutdown && drv->shutdown))