perf record: Force enable --timestamp-filename when --switch-output is provided

Without this patch, the last output doesn't have timestamp appended if
--timestamp-filename is not explicitly provided. For example:

  # perf record -a --switch-output &
  [1] 11224
  # kill -s SIGUSR2 11224
  [ perf record: dump data: Woken up 1 times ]
  # [ perf record: Dump perf.data.2015122622372823 ]

  # fg
  perf record -a --switch-output
  ^C[ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.027 MB perf.data (540 samples) ]

  # ls -l
  total 836
  -rw------- 1 root root  33256 Dec 26 22:37 perf.data   <---- *Odd*
  -rw------- 1 root root 817156 Dec 26 22:37 perf.data.2015122622372823

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1461178794-40467-5-git-send-email-wangnan0@huawei.com


Signed-off-by: He Kuang <hekuang@huawei.com>
[ Updated man page, that also got an entry for --timestamp-filename ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>