perf stat record: Keep sample_type 0 for pipe session

perf stat record: Keep sample_type 0 for pipe session

For pipe sessions we need to keep sample_type zero, because script's
perf_evsel__check_attr is triggered by sample_type != 0, and the check
would fail on stat session.

I was tempted to keep it zero unconditionally, but the pipe session is
sufficient. In perf.data session we are guarded by HEADER_STAT feature.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1452028152-26762-2-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>