md: Free writes_pending in md_stop

dm raid calls md_stop to stop the raid device. It needs to
free the writes_pending here.
Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>

drivers/md/md.c

@@ -6265,6 +6265,7 @@ void md_stop(struct mddev *mddev)
 	 */
 	__md_stop_writes(mddev);
 	__md_stop(mddev);
+	percpu_ref_exit(&mddev->writes_pending);
 	percpu_ref_exit(&mddev->active_io);
 	bioset_exit(&mddev->bio_set);
 	bioset_exit(&mddev->sync_set);