[kpartx] fix partition calculations of DASD partitions

[kpartx] fix partition calculations of DASD partitions

This patch fixes the offset and length calcuations for DASD partitions
with either CMS (without explicit offset), the old linux disk layout or
no partition information at all if the devices are using a blocksize
bigger than 512B.
In the cases mentioned above the offset was normalized (to the number of
512B sectors) twice and the size reduced by the normalized offset and
again by the double normalized offset later.
This leads to kpartx defining the wrong limits in these cases.

Signed-off-by: Stefan Bader <shbader@de.ibm.com>