Upstream: add missing thread_exit()

Undo FreeBSD wrapper for thread_create() added to call thread_exit.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #10314

module/zfs/vdev_initialize.c

@@ -558,6 +558,8 @@ vdev_initialize_thread(void *arg)
 	vd->vdev_initialize_thread = NULL;
 	cv_broadcast(&vd->vdev_initialize_cv);
 	mutex_exit(&vd->vdev_initialize_lock);
+
+	thread_exit();
 }
 
 /*