fdisk: fix io_size usage in new API

fdisk: fix io_size usage in new API

properly implemented fdisk_dev_has_topology() requires optimal
I/O size to detect that the device provides topology.

Unfortunately, currently used cxt->io_size maybe overwritten in
__discover_topology() to min_io_size.

This patch introduces cxt->optimal_io_size and keeps it independent on
cxt->io_size. The cxt->io_size is I/O size used by fdisk for alignment
calculation.

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