udev: probe GPT sector size and trigger loop device on mismatch

udev: probe GPT sector size and trigger loop device on mismatch

When the GPT partition table uses a different sector size than the
device's native block size (e.g. 512-byte GPT on a 2048-byte CD-ROM
booted via El Torito), the kernel cannot parse the partition table.

Probe the GPT sector size upfront and configure blkid with the correct
value so it always finds the partition table. If a sector size mismatch
is detected, trigger a loop device to re-expose the device with the
correct sector size and skip root partition discovery on the original
device — it will happen on the loop device instead.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>