nbd: fix use after free on module unload

list_for_each_entry() isn't super safe if we're freeing the objects
while we traverse the list.  Also don't bother taking the extra
reference, the module refcounting stuff will save us from having anybody
messing with the device while we're trying to unload.
Reported-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/nbd.c

@@ -2090,7 +2090,6 @@ static int nbd_exit_cb(int id, void *ptr, void *data)
 	struct list_head *list = (struct list_head *)data;
 	struct nbd_device *nbd = ptr;
 
-	refcount_inc(&nbd->refs);
 	list_add_tail(&nbd->list, list);
 	return 0;
 }
@@ -2106,11 +2105,12 @@ static void __exit nbd_cleanup(void)
 	idr_for_each(&nbd_index_idr, &nbd_exit_cb, &del_list);
 	mutex_unlock(&nbd_index_mutex);
 
-	list_for_each_entry(nbd, &del_list, list) {
-		if (refcount_read(&nbd->refs) != 2)
+	while (!list_empty(&del_list)) {
+		nbd = list_first_entry(&del_list, struct nbd_device, list);
+		list_del_init(&nbd->list);
+		if (refcount_read(&nbd->refs) != 1)
 			printk(KERN_ERR "nbd: possibly leaking a device\n");
 		nbd_put(nbd);
-		nbd_put(nbd);
 	}
 
 	idr_destroy(&nbd_index_idr);