[$] Mount notifications

There are a handful of extensions to the 'new' mount API that Christian
Brauner wanted to discuss as part of a filesystem session at
the
2024 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 mount
operations (e.g. mount and unmount) that happen in the system, especially
for tools like container
managers or systemd.