Fix two issues:
- the per-cpu stat flush is unnecessary, nobody uses per-cpu stat except
  sum it to global stat. We can do the calculation there. The flush just
  wastes cpu time.
- some fields are signed int/s64. I don't see the point.
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NShaohua Li <shli@fb.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>