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