drbd: fix drbd_destroy_device reference count updates

drbd_destroy_device means to give up reference counts
on the connection(s) reachable via the peer_device(s).

It must not do that by iterating via device->resource->connections,
resource and connections may have already been disassociated
by drbd_free_resource, and we'd leak connection refs.

Instead, iterate via device->peer_devices->connection.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_main.c

@@ -2160,7 +2160,7 @@ void drbd_destroy_device(struct kref *kref)
 {
 	struct drbd_device *device = container_of(kref, struct drbd_device, kref);
 	struct drbd_resource *resource = device->resource;
-	struct drbd_connection *connection;
+	struct drbd_peer_device *peer_device, *tmp_peer_device;
 
 	del_timer_sync(&device->request_timer);
 
@@ -2191,12 +2191,16 @@ void drbd_destroy_device(struct kref *kref)
 	put_disk(device->vdisk);
 	blk_cleanup_queue(device->rq_queue);
 	kfree(device->rs_plan_s);
-	kfree(first_peer_device(device));
+
+	/* not for_each_connection(connection, resource):
+	 * those may have been cleaned up and disassociated already.
+	 */
+	for_each_peer_device_safe(peer_device, tmp_peer_device, device) {
+		kref_put(&peer_device->connection->kref, drbd_destroy_connection);
+		kfree(peer_device);
+	}
 	memset(device, 0xfd, sizeof(*device));
 	kfree(device);
-
-	for_each_connection(connection, resource)
-		kref_put(&connection->kref, drbd_destroy_connection);
 	kref_put(&resource->kref, drbd_destroy_resource);
 }