loop-util: don't reuse partition fd when partscan needed

loop-util: don't reuse partition fd when partscan needed

Some devices (e.g. android phones running pmOS) cannot have their OEM
partition table altered without breaking the firmware, so the distros's
partitions live inside a nested GPT carved into one of the OEM
partitions. Exposing these subpartitions requires wrapping the outer
partition in a loop device with partscan enabled, since the kernel does
not go into nested partition tables.

systemd already detects this case in udev-builtin-blkid
(ID_PART_GPT_AUTO_ROOT_DISK_NEEDS_LOOP) and acts on with
systemd-loop@.service, but this fails towards the end.
loop_device_make_internal has an optimization where if the input is
already a block device with a matching sector size, it skips creating
a loop and just hands back the original fd. That's fine for whole disks
but wrong for partitions, which don't support partscan, so this causes
dissect_image to fail with EPROTONOSUPPORT.

This patch changes the behavior to only take the shortcut when the input
is a whole disk, or when partscan was not requested.

Co-Authored-By: Clayton Craft <clayton@craftyguy.net>