perf tools: Use scandir() to replace readdir()

In perf_event__synthesize_threads() perf goes through all proc files
serially by readdir.

scandir() does a snapshoot of /proc, which is multithreading friendly.

It's possible that some threads which are added during event synthesize.
But the number of lost threads should be small.  They should not impact
the final analysis.

Signed-off-by: Kan Liang <kan.liang@intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Lukasz Odzioba <lukasz.odzioba@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1504806954-150842-3-git-send-email-kan.liang@intel.com


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