drivers, connector: convert cn_callback_entry.refcnt from atomic_t to refcount_t

atomic_t variables are currently used to implement reference
counters with the following properties:
- counter is initialized to 1 using atomic_set()
- a resource is freed upon counter reaching zero
- once counter reaches zero, its further
increments aren't allowed
- counter schema uses basic atomic operations
(set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable cn_callback_entry.refcnt is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <[e-mail]>
Reviewed-by: David Windsor <[e-mail]>
Reviewed-by: Hans Liljestrand <[e-mail]>
Signed-off-by: Elena Reshetova <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/connector/cn_queue.c

···
 		return NULL;
 	}
 
-	atomic_set(&cbq->refcnt, 1);
+	refcount_set(&cbq->refcnt, 1);
 
 	atomic_inc(&dev->refcnt);
 	cbq->pdev = dev;
···
 
 void cn_queue_release_callback(struct cn_callback_entry *cbq)
 {
-	if (!atomic_dec_and_test(&cbq->refcnt))
+	if (!refcount_dec_and_test(&cbq->refcnt))
 		return;
 
 	atomic_dec(&cbq->pdev->refcnt);

drivers/connector/connector.c

···
 	spin_lock_bh(&dev->cbdev->queue_lock);
 	list_for_each_entry(i, &dev->cbdev->queue_list, callback_entry) {
 		if (cn_cb_equal(&i->id.id, &msg->id)) {
-			atomic_inc(&i->refcnt);
+			refcount_inc(&i->refcnt);
 			cbq = i;
 			break;
 		}

include/linux/connector.h

···
 #define __CONNECTOR_H
 
 
-#include <linux/atomic.h>
+#include <linux/refcount.h>
 
 #include <linux/list.h>
 #include <linux/workqueue.h>
···
 
 struct cn_callback_entry {
 	struct list_head callback_entry;
-	atomic_t refcnt;
+	refcount_t refcnt;
 	struct cn_queue_dev *pdev;
 
 	struct cn_callback_id id;