perf session: Get rid of file_size variable

perf session: Get rid of file_size variable

It's not needed and removing it makes the code a little simpler for the
upcoming changes.

It's safe to replace file_size with data_size, because the
perf_data__size() value is never smaller than data_offset + data_size.

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/20190110101301.6196-3-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>