blkzone: fix whole device detection

blkzone: fix whole device detection

Matias Bjørling wrote:
  The current detection code for chunk size assumes that the
  underlying device is a device that uses the minor device id
  as the partition id, and that the whole device has minor id 0.
  This is not true for example null_blk and nvme device drivers.

Let's use sysfs_devno_to_wholedisk() to get whole-disk devno.

Addresses: https://github.com/karelzak/util-linux/pull/646
Reported-by: Matias Bjørling matias.bjorling@wdc.com
Signed-off-by: Karel Zak <kzak@redhat.com>