perf bench: Flush stdout before starting bench suite

perf bench prints header message for bench suite before starting the
benchmark.  However if the stdout is redirected to a file and bench
suite forks child processes this (and possibly other debugging
messages too) will be repeated multiple times.

  $ perf bench sched messaging
  # Running sched/messaging benchmark...
  # 20 sender and receiver processes per group
  # 10 groups == 400 processes run

       Total time: 0.100 [sec]

  $ perf bench sched messaging > result.txt
  $ wc -l result.txt
  391

In this file, there were so many "Running sched/messaging benchmark..."
lines.  This was because stdout is converted to fully-buffered due to
the redirection and inherited child processes.  Other lines are printed
after reaping all those tasks.

So fix it by flushing stdout before starting bench suites.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Hitoshi Mitake <h.mitake@gmail.com>
Cc: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1357637966-8216-1-git-send-email-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>