Add missed thread_exit() to vdev_{autotrim,rebuild}_thread

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10668

module/zfs/vdev_rebuild.c

@@ -911,6 +911,8 @@ vdev_rebuild_thread(void *arg)
 	spa_config_exit(spa, SCL_CONFIG, FTAG);
 
 	cv_broadcast(&vd->vdev_rebuild_cv);
+
+	thread_exit();
 }
 
 /*

module/zfs/vdev_trim.c

@@ -1411,6 +1411,8 @@ vdev_autotrim_thread(void *arg)
 	vd->vdev_autotrim_thread = NULL;
 	cv_broadcast(&vd->vdev_autotrim_cv);
 	mutex_exit(&vd->vdev_autotrim_lock);
+
+	thread_exit();
 }
 
 /*