fix crash in table histogram

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>
fix deadlock when exit in non-mainthread when function tracing

we cannot use wake_up_process in kp_exit for non-mainthread,
since ktap function maybe called from sched code(when function tracer),
which will make deadlock for re-grab runqueue lock.

Now we use a stupid looping in ktap mainthread, to wait for
ks->stop to be 1, this will not deadlock anymore.

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>