nullb: use blk_queue_set_zoned() to setup zoned devices

Use blk_queue_set_zoned() to set a nullb device zone model instead of
directly assigning the device queue zoned limit. This initialization of
the devicve zoned model as well as the setup of the queue flag
QUEUE_FLAG_ZONE_RESETALL and of the device queue elevator feature are
moved from null_init_zoned_dev() to null_register_zoned_dev() so that
the initialization of the queue limits is done when the gendisk of the
nullb device is available.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@edc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>