driver core: fix namespace issue with devices assigned to classes

- uses a kset in "struct class" to keep track of all directories
    belonging to this class
  - merges with the /sys/devices/virtual logic.
  - removes the namespace-dir if the last member of that class
    leaves the directory.

There may be locking or refcounting fixes left, I stopped when it seemed
to work with network and sound modules. :)

From: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/base/class.c

@@ -145,6 +145,7 @@ int class_register(struct class * cls)
 	INIT_LIST_HEAD(&cls->children);
 	INIT_LIST_HEAD(&cls->devices);
 	INIT_LIST_HEAD(&cls->interfaces);
+	kset_init(&cls->class_dirs);
 	init_MUTEX(&cls->sem);
 	error = kobject_set_name(&cls->subsys.kset.kobj, "%s", cls->name);
 	if (error)
@@ -163,7 +164,6 @@ int class_register(struct class * cls)
 void class_unregister(struct class * cls)
 {
 	pr_debug("device class '%s': unregistering\n", cls->name);
-	kobject_unregister(cls->virtual_dir);
 	remove_class_attrs(cls);
 	subsystem_unregister(&cls->subsys);
 }