null_blk: Add zone_nr_conv to features

For a null_blk device with zoned mode enabled, the number of
conventional zones can be configured through configfs with the
zone_nr_conv parameter. Add this missing parameter in the features
string.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>