zbd: remove is_zone_open() helper

zbd: remove is_zone_open() helper

The helper function is_zone_open() is useless as a each zone has an open
flag indicating if it is part of the array of open zones. Remove this
function code and use the zone open flag in zbd_open_zone().

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Link: https://lore.kernel.org/r/20211214012413.464798-5-damien.lemoal@opensource.wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>