Fix issue #1533.

Currently, there exist a race condition between perf_reader
buffer munmap and read if they are happening in two different
threads, crash is possible as in issue #1533.

          thread 1                    thread 2
          perf_reader_event_read      ...
                                      detach_probe
                                      munmap
          access ring buffer

detach_probe may happen as part of bpf object exit cleanup process
at which point thread 1 is still alive. In this case, accessing
ring buffer may cause segfault since the original mmap'ed memory
is not available any more.

It is hard to fix such races outside bcc since user
calls kprobe_poll which has valid BPF object when it is called,
but race happens inside the kprobe_poll.

This patch adds a state of the ring buffer and the read will
not happen once the state comes to the munmap.
Signed-off-by: Yonghong Song <yhs@fb.com>