samples: Use KSYM_NAME_LEN for kprobes

It is better and enough to use KSYM_NAME_LEN for kprobes
in samples, no need to define and use the other values.

Link: https://lkml.kernel.org/r/1654651402-21552-1-git-send-email-yangtiezhu@loongson.cn

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>