tools/uma/smrstress: fix kthread exit

By convention, kernel threads must call kthread_exit() instead of
blindly returning from the thread function.  We have some safety measure
in fork_exit(), which checks for the P_KPROC p_flag and does
kthread_exit() for kernel thread that forgot to do it itself.

But this workaround only works for kernel threads belonging to the
kernel process.  If a kernel thread is attached to the normal process
with live userspace, and does not call kthread_exit(), then the
workaround is not activated, and for amd64 at least, the return from the
thread function/fork_exit() results in the return to userspace with the
copy of frame from the thread that did kthread_add().

Practically for smrstress, this destroys the user stack of the still
active frame in the other thread, which was the caller of kthread_add().

Fix it by adding kthread_exit() to the thread function.

Reported and tested by:	pho
Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D35999

tools/uma/smrstress/smrstress.c

@@ -123,6 +123,7 @@ smrs_thread(void *arg)
 	else
 		smrs_read();
 	atomic_add_int(&smrs_completed, 1);
+	kthread_exit();
 }
 
 static void