repart: Reuse the backing fd for fdisk

repart: Reuse the backing fd for fdisk

Because fdisk_assign_device tries to open block devices with O_EXCL, when it
does it blocks cryptsetup from using partition block devices for the same
disk.

Since we already have a file descriptor for the device, we can just share it
and use fdisk_assign_device_by_fd instead.

This requires at least libfdisk 2.35 (part of util-linux) which was
released in 2020.