zbd: allow zonemode=zbd with regular files by emulating zones

zbd: allow zonemode=zbd with regular files by emulating zones

Currently when using zonemode=zbd and running against a regular file,
fio will fail with:
fio: file hash not empty on exit

Treat regular files just like how we treat regular (non-zoned) block
devices: return ZBD_NONE and let zbd.c emulate zones inside the regular
file/block device.

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