kprobes: Lock kprobe_mutex while showing kprobe_blacklist

Lock kprobe_mutex while showing kprobe_blacklist to prevent updating the
kprobe_blacklist.

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/20200505134059.571125195@linutronix.de