null_blk: Add conventional zone configuration for zoned support

Allow the creation of conventional zones by adding the zone_nr_conv
configuration attribute. This new attribute is used only for zoned devices
and indicates the number of conventional zones to create. The default value
is 0. Since host-managed zoned block devices must always have at least one
sequential zone, if the value of zone_nr_conv is larger than or equal to
the total number of zones of the device nr_zones, zone_nr_conv is
automatically changed to nr_zones - 1.

Reviewed-by: Matias Bjorling <matias.bjorling@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Masato Suzuki <masato.suzuki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>