dm-zoned: cleanup dmz_fixup_devices

Use the bdev based helpers where applicable and move the zoned_dev
into the scope where it is actually used.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20220706070350.1703384-15-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>