fdisk: fix -b <sectorsize>

fdisk: fix -b <sectorsize>

The old fdisk (<2.17) does not differentiate between logical and
physical sector size, it uses the <sectorsize> for everything.

Now, we have logical and physical sectors size, but the -b option
changes the logical size only. The second bug is that "fdisk -b <sz>"
does not read topology information (it means that all I/O limits and
physical sector size are 512 (default).

The backwardly compatible bug fix is to override both sizes, logical
and physical if "-b" is used.

In future we can add a special option for physical size only.

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