samples/kprobes: Fix typo in handler_post()

It should use post_handler instead of pre_handler in handler_post().

As Joe Perches suggested, it would be better to use pr_fmt and remove
all the embedded pre/post/fault strings. This would change the style of
the output through.

Fixes: e16c5dd5

 ("samples/kprobes: Add s390 case in kprobe example module")
Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Acked-by: Marc Koderer <marc@koderer.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/1621218083-23519-3-git-send-email-yangtiezhu@loongson.cn
Signed-off-by: Jonathan Corbet <corbet@lwn.net>