perf session: Add reader__process_events function

perf session: Add reader__process_events function

The reader object is defined by file's fd, data offset and data size.

Now we can simply define a reader object for an arbitrary file data
portion and pass it to reader__process_events().

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190110101301.6196-7-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>