md: notify mdstat waiters of level change

Level modifications change the output of mdstat.  The mdmon manager
thread is interested in these events for external metadata management.
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/md/md.c

@@ -3075,6 +3075,7 @@ level_store(mddev_t *mddev, const char *buf, size_t len)
 	set_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
 	md_wakeup_thread(mddev->thread);
 	sysfs_notify(&mddev->kobj, NULL, "level");
+	md_new_event(mddev);
 	return rv;
 }