fix event tracing in timer closure

Currently ktap disallow tracing event in timer callback closure,
that will corrupt ktap_state and ktap stack, because timer closure
and event closure use same irq percpu ktap_state and stack.
We can use a different percpu ktap_state and stack for timer purpuse,
but that's don't bring any big value with cost on memory consuming.

So just simply disable tracing in timer closure,
introduce ktap_in_tracing percpu variable.