libfdisk: fix sectorbuffer usage in dos driver

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: fix cylinders and sector buffer usage

 - sector buffer has to be correctly deallocated
 - round to cylinders when use cylinder units in dos driver

Signed-off-by: Karel Zak <kzak@redhat.com>
tests: refresh fdisk tests