ftrace/kprobe: Show the maxactive number on kprobe_events

Show maxactive parameter on kprobe_events.
This allows user to save the current configuration and
restore it without losing maxactive parameter.

Link: http://lkml.kernel.org/r/4762764a-6df7-bc93-ed60-e336146dce1f@gmail.com
Link: http://lkml.kernel.org/r/158503528846.22706.5549974121212526020.stgit@devnote2

Cc: stable@vger.kernel.org
Fixes: 696ced4f

 ("tracing/kprobes: expose maxactive for kretprobe in kprobe_events")
Reported-by: Taeung Song <treeze.taeung@gmail.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>