zbd: allow an unlimited global max open zones limit

zbd: allow an unlimited global max open zones limit

Commit 219c662d3b12 ("zbd: introduce per job maximum open zones limit")
Introduced a global max open zones limit stored in zbd_info->max_open_zones.

This commit also removed the following check from zbd_open_zone():

if (!td->o.max_open_zones)
        return true;

Before the commit in question, if td->o.max_open_zones == 0, zbd_open_zone()
would not track the zone as open in the zbd_info->open_zones array.

After the commit in question, zbd_open_zone() would always track the zone
as open in the zbd_info->open_zones array, regardless if a --max_open_zones
limit was set or not.

Change the initialization of zbd_info->max_open_zones to yet again allow
unlimited max open zones. If zbd_info->max_open_zones is unlimited,
we do not track the open zones in the zbd_info->open zone array.

Not tracking open zones reduces fio overhead for testing the performance of
zoned block devices that do not have a limit on the maximum number of open
zones.

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>