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_removal.c

@@ -1595,6 +1595,8 @@ spa_vdev_remove_thread(void *arg)
 		ASSERT0(range_tree_space(svr->svr_allocd_segs));
 		vdev_remove_complete(spa);
 	}
+
+	thread_exit();
 }
 
 void