ublk_drv: use flexible-array member instead of zero-length array

Eliminate the following coccicheck warning:
./drivers/block/ublk_drv.c:127:16-19: WARNING use flexible-array member instead

Signed-off-by: Yushan Zhou <katrinzhou@tencent.com>
Link: https://lore.kernel.org/r/20221018100132.355393-1-zys.zljxml@gmail.com
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>