libfdisk: don't use off_t in public API

libfdisk: don't use off_t in public API

It's better to use exact and explicitly defined types (e.g. uint64_t)
rather than something like off_t to make code more portable.

[reported with gcc (Ubuntu 4.4.3-4ubuntu5.1) 4.4.3)]

The patch also fixes one debug message.

Reported-by: Benno Schulenberg <bensberg@justemail.net>
Signed-off-by: Karel Zak <kzak@redhat.com>