[$] Mounting images inside a user namespace

There has long been a desire to enable users to mount filesystem images withoutrequiring privileges, but the securityimplications of allowing it are seriously concerning. Few, if any, kernelfilesystems are hardened against maliciously crafted images, after all.Lennart Poettering led a filesystem session at the 2023 Linux Storage, Filesystem,Memory-Management and BPF Summit where he presented a possible pathforward.