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>

include/linux/device.h

@@ -181,10 +181,9 @@ struct class {
 	struct list_head	children;
 	struct list_head	devices;
 	struct list_head	interfaces;
+	struct kset		class_dirs;
 	struct semaphore	sem;	/* locks both the children and interfaces lists */
 
-	struct kobject		*virtual_dir;
-
 	struct class_attribute		* class_attrs;
 	struct class_device_attribute	* class_dev_attrs;
 	struct device_attribute		* dev_attrs;