USN-6882-2: Cinder regression

USN-6882-1 fixed vulnerabilities in Cinder. The update caused a regression
in certain environments due to incorrect privilege handling. This update
fixes the problem.

We apologize for the inconvenience.

Original advisory details:

Martin Kaesberger discovered that Cinder incorrectly handled QCOW2 image
processing. An authenticated user could use this issue to access arbitrary
files on the server, possibly exposing sensitive information.