block: ublk_drv: don't consider flush request in map/unmap io

There isn't data in request of REQ_OP_FLUSH always, so don't consider
it in both ublk_map_io() and ublk_unmap_io().

Reviewed-by: Ziyang Zhang <ZiyangZhang@linux.alibaba.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>