[$] Monitoring mount operations

Amir Goldstein kicked off a session on monitoring mounts at the 2023 Linux Storage, Filesystem, Memory-Management and BPF Summit. In particular, there are problems when trying to efficiently monitor "a very large number of mounts in a mount namespace"; some user-space programs need an accurate view of the mount tree without having to constantly parse /proc/mounts or the like. There are a number of questions to be answered, including what the API should look like and what entity should be watched in order to get notifications of new mount operations.