disk/loopback: Reference tracking for the loopback

disk/loopback: Reference tracking for the loopback

It was possible to delete a loopback while there were still references
to it. This led to an exploitable use-after-free.

Fixed by implementing a reference counting in the grub_loopback struct.

Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Gbp-Pq: Topic cve-2025-jan
Gbp-Pq: Name disk-loopback-Reference-tracking-for-the-loopback.patch