fix a kernel crash in histogram

forgot to set value of table array in thr data

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

This script is purpose for output average execution time of every softirq

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>
minor fix in scripts/function_time.kp

check self[tid()] as nil, instead of 0

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

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>
print ktap string properly in dump_function