libfdisk: (dos) calculation of total size based on CHS in check function

Many people report a problem with the message if (cylinders <= 1024 &&
start != total) fprintf(stderr, _("Partition %d: previous sectors %d
disagrees with " "total %d\n"), n, start, total);

This message comes from the fact that the previous code caluclates the
total number of sectors in the wrong way.

The formula should be total = (real_c * heads + h) * sectors + real_s;

As a Cylinder consists of some heads * sectors A Head consists of some
sectors and finally sectors are the smallest unit

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: (dos) use check() for dos-comaptible mode only

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