perf data: Add perf_data__open_dir_data function

Add perf_data__open_dir_data to open files inside 'struct perf_data'
path directory:

   static int perf_data__open_dir(struct perf_data *data);

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20190224190656.30163-10-jolsa@kernel.org


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