fix userspace malloc size too small

g_idstr may contains all tracepoints str, which is bigger than 3000,
so malloc it with 4096 bytes would be enough.

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>
add sample script event_trigger.kp

This script demos simplest event trigger in ktap,
there already have ftrace event trigger in Linux kernel, but
generic event trigger is still not supported at present.

In ktap, support event trigger is so simple, not even any new
code in kerne, just a script.

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