udev: also trigger loop device for boot disk when partition scanning is unsupported

udev: also trigger loop device for boot disk when partition scanning is unsupported

Previously, probe_gpt_sector_size_mismatch() would bail out early when
the GPT sector size matched the device sector size. However, some
devices (e.g. certain CD-ROM drives) do not support kernel partition
scanning even when sector sizes match. In that case, the kernel still
cannot parse the partition table, and we need to set up a loop device to
expose the partitions — just as we do for the sector size mismatch case.

Check blockdev_partscan_enabled() when sector sizes match, and only skip
the boot partition check if partition scanning is actually supported.

Also rename the function, udev property, and log messages to reflect the
broader scope:

- probe_gpt_sector_size_mismatch() -> probe_gpt_boot_disk_needs_loop()
- ID_PART_GPT_AUTO_ROOT_DISK_SECTOR_SIZE_MISMATCH -> ID_PART_GPT_AUTO_ROOT_DISK_NEEDS_LOOP