libfdisk: (dos) fix off-by-one in maximum last sector calculation

libfdisk: (dos) fix off-by-one in maximum last sector calculation

The get_disk_ranges() function incorrectly capped the last usable
sector at UINT_MAX, which could cause an overflow when calculating
partition size for MBR partition tables.

MBR stores partition size as a 32-bit value with maximum UINT_MAX.
The partition size is calculated as: size = stop - start + 1

For a partition starting at sector 0:
- If stop = UINT_MAX: size = UINT_MAX + 1 (overflow!)
- If stop = UINT_MAX - 1: size = UINT_MAX (correct maximum)

This fixes the inconsistency where dos_init() correctly warns about
disks larger than UINT_MAX sectors (2TiB - 512 bytes for 512-byte
sectors), but get_disk_ranges() allowed creating partitions that
would overflow the 32-bit size field.

Addresses: https://issues.redhat.com/browse/RHEL-122367
Signed-off-by: Karel Zak <kzak@redhat.com>