driver core: class: properly reference count class_dev_iter()

When class_dev_iter is initialized, the reference count for the subsys
private structure is incremented, but never decremented, causing a
memory leak over time. To resolve this, save off a pointer to the
internal structure into the class_dev_iter structure and then when the
iterator is finished, drop the reference count.

Reported-and-tested-by: [e-mail]
Fixes: 7b884b7f24b4 ("driver core: class.c: convert to only use class_to_subsys")
Reported-by: Mirsad Goran Todorovac <[e-mail]>
Cc: Alan Stern <[e-mail]>
Acked-by: Rafael J. Wysocki <[e-mail]>
Tested-by: Mirsad Goran Todorovac <[e-mail]>
Link: https://lore.kernel.org/r/2023051610-stove-condense-9a77@gregkh
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/class.c

···
 		start_knode = &start->p->knode_class;
 	klist_iter_init_node(&sp->klist_devices, &iter->ki, start_knode);
 	iter->type = type;
+	iter->sp = sp;
 }
 EXPORT_SYMBOL_GPL(class_dev_iter_init);
 
···
 void class_dev_iter_exit(struct class_dev_iter *iter)
 {
 	klist_iter_exit(&iter->ki);
+	subsys_put(iter->sp);
 }
 EXPORT_SYMBOL_GPL(class_dev_iter_exit);
 

include/linux/device/class.h

···
 struct class_dev_iter {
 	struct klist_iter		ki;
 	const struct device_type	*type;
+	struct subsys_private		*sp;
 };
 
 int __must_check class_register(const struct class *class);