options: Add zonecapacity option for zonemode=zbd

options: Add zonecapacity option for zonemode=zbd

To test zone capacity handling by fio using regular block devices, add
zonecapacity option for zonemode=zbd. This new option allows specifying a
zone capacity smaller than the zone size. Similarly to the zonesize
option, the zonecapacity defined value applies to all emulated zones of
the device.

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>