vmbus: unregister device_obj->channels_kset

Without the patch, a device can't be thoroughly destroyed, because
vmbus_device_register() -> kset_create_and_add() still holds a reference
to the hv_device's device.kobj.

Signed-off-by: Dexuan Cui <[e-mail]>
Cc: Stephen Hemminger <[e-mail]>
Fixes: c2e5df616e1a ("vmbus: add per-channel sysfs info")
Signed-off-by: K. Y. Srinivasan <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/hv/vmbus_drv.c

···
 	pr_debug("child device %s unregistered\n",
 		dev_name(&device_obj->device));
 
+	kset_unregister(device_obj->channels_kset);
+
 	/*
 	 * Kick off the process of unregistering the device.
 	 * This will call vmbus_remove() and eventually vmbus_device_release()