samples/kprobes: Add __kprobes and NOKPROBE_SYMBOL() for handlers.

Add __kprobes and NOKPROBE_SYMBOL() for sample kprobe handlers.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200505134059.878578033@linutronix.de