perf evsel: Print state of perf_event_attr.write_backward

perf evsel: Print state of perf_event_attr.write_backward

Now we can see if it is set when using verbose mode in various tools,
such as 'perf test':

  # perf test -vv back
  45: Test backward reading from ring buffer                   :
  --- start ---
  <SNIP>
  ------------------------------------------------------------
  perf_event_attr:
    type                             2
    size                             112
    config                           0x98
    { sample_period, sample_freq }   1
    sample_type                      IP|TID|TIME|CPU|PERIOD|RAW
    disabled                         1
    mmap                             1
    comm                             1
    task                             1
    sample_id_all                    1
    exclude_guest                    1
    mmap2                            1
    comm_exec                        1
    write_backward                   1
  ------------------------------------------------------------
  sys_perf_event_open: pid 20911  cpu -1  group_fd -1  flags 0x8
  <SNIP>
  ---- end ----
  Test backward reading from ring buffer: Ok
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-kxv05kv9qwl5of7rzfeiiwbv@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>