repart: fix sector size handling

repart: fix sector size handling

This queries the sector size from libfdisk instead of assuming 512, and
uses that when converting from bytes to the offset/size values libfdisk
expects.

This is an alternative to Tom Yan's #21823, but prefers using libfdisk's
own ideas of the sector size instead of going directly to the backing
device via ioctls. (libfdisk can after all also operate on regular
files, where the sector size concept doesn't necessarily apply the same
way.)

This also makes the "grain" variable, i.e. how we'll align the
partitions. Previously this was hardcoded to 4K, and that still will be
the minimum grain we use, but should the sector size be larger than that
we'll use the next multiple of the sector size instead.