zbd: add zbd_zone_remainder() helper function

zbd: add zbd_zone_remainder() helper function

Add the helper function zbd_zone_remainder(), which returns the number
of bytes that are still available for writing before the zone gets full.
Use this function to improve readability. It will also be used in the
following patch.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Tested-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>