ftrace/kprobes: selftests: Check kretprobe maxactive is supported

Check the kretprobe maxactive is supported by kprobe_events
interface. To ensure the kernel feature, this changes ftrace
README to describe it.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>