spl-taskq: Make sure thread tsd hash entry is cleared

Like any other thread created by thread_create() we need to call
thread_exit() to properly clean it up.  In particular, this ensures the
tsd hash for the thread is cleared.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #11512

module/os/linux/spl/spl-taskq.c

@@ -995,6 +995,7 @@ error:
 	spin_unlock_irqrestore(&tq->tq_lock, flags);
 
 	tsd_set(taskq_tsd, NULL);
+	thread_exit();
 
 	return (0);
 }