[$] Mount notifications

There are a handful of extensions to the "new" mount API that ChristianBrauner wanted to discuss as part of a filesystem session at the2024 Linux Storage,Filesystem, Memory Management, and BPF Summit. In the session, though,the only one that he got to was a followup to last year's discussion on mount-operation monitoring.There is a need for user-space programs to be able to follow mountoperations (e.g. mount and unmount) that happen in the system, especiallyfor tools like containermanagers or systemd.