sfdisk: setting default geometry values

sfdisk: setting default geometry values

The cfdisk and fdisk set defaults for heads and sectors in order to handle a
situation when the get_geometry() ends up with zeroes for the disk geometry for
device-mapper devices. This patch add same functionally to sfdisk.

You know that a dmsetup setgeometry of the the dm device can be used to
circumvent the issue but users may not immediately realize they need to do
that.