Make rbd engine use aio flush

The fio's option "fsync" can make rbd engine issue sync request. In practice,
we would like to use aio_rbd_flush to make it non-block. So it will be more
reliable to use fio to evaluate actual io scene like qemu.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>