driver core: delete virtual directory on class_unregister()

Class virtual directory is created as the need arises.
But it is not deleted when the class is unregistered.

Signed-off-by: Akinobu Mita <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/class.c

···
 void class_unregister(struct class * cls)
 {
 	pr_debug("device class '%s': unregistering\n", cls->name);
+	if (cls->virtual_dir)
+		kobject_unregister(cls->virtual_dir);
 	remove_class_attrs(cls);
 	subsystem_unregister(&cls->subsys);
 }