udev: only trigger the boot-disk loop device for optical drives

udev: only trigger the boot-disk loop device for optical drives

probe_gpt_boot_disk_needs_loop() sets ID_PART_GPT_AUTO_ROOT_DISK_NEEDS_LOOP
for any whole disk that holds the boot ESP/XBOOTLDR but whose partition table
the kernel cannot parse. Until now the udev rule turned that into a
systemd-loop@.service for every block device.

That is too broad: device-mapper devices also report kernel partition
scanning as disabled, but their partitions are managed in userspace by kpartx
(see 66-kpartx.rules). Setting up a loop device on top of them re-exposes the
same partition table a second time and only causes trouble.

Restrict the rule to optical drives, the one class that genuinely needs a
kernel-side loop device (El Torito GPT sector size mismatch, or drives that do
not support partition scanning) and that has no userspace partition manager of
its own.

Co-developed-by: Claude Fable 5 <noreply@anthropic.com>