libfdisk: (dos) count fisrt usable LBA to total number of used sectors

The check() function counts number of used sectors, but it counts only
partitions. This is mistake, the area before the first partition is
also "used" place (boot loaders or we have to align the first partition
to disk I/O limits, etc).

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