perf tools: Do not seek in pipe fd during tracing data processing

There's no need to set 'fd' position in pipe mode, the file descriptor
is already in proper place. Moreover the lseek will fail on pipe
descriptor and that's why it's been working properly.

I was tempted to remove the lseek calls completely, because it seems
that tracing data event was always synthesized only in pipe mode, so
there's no need for 'file' mode handling. But I guess there was a reason
behind this and there might (however unlikely) be a perf.data that we
could break processing for.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Khuong <pvk@pvk.ca>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200507095024.2789147-3-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>