[$] Two VFS topics

Two different topics concerning the virtual filesystem (VFS) layer were thesubject of a session led by VFS co-maintainer Christian Brauner at the2023 Linux Storage, Filesystem,Memory-Management and BPF Summit. As might be guessed, it was afilesystem-track session; Brauner had three separate items he planned onbringing up, but the discussion on the first two consumed the wholehalf-hour-and then some. A mechanism to avoid media-change races whenmounting loop (or loopback) and other devices was disposed of fairlyquickly, but the discussion around the mount-beneath feature went on at length.