ublk: pass NULL to blk_mq_alloc_disk() as queuedata

queuedata is not referenced in ublk_drv and we can use driver_data
instead. Pass NULL to blk_mq_alloc_disk() as queuedata while allocating
ublk's gendisk.

Signed-off-by: Ziyang Zhang <ZiyangZhang@linux.alibaba.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230207070839.370817-4-ZiyangZhang@linux.alibaba.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>