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>
67f70f70a3