tracing/function: Cleanup for function tracer

We can directly use %pf input format instead of kallsyms_lookup()
and %s input format

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>