[$] Topics from the virtual filesystem layer

In the first filesystem-track session at the 2025 Linux Storage,
Filesystem, Memory Management, and BPF Summit (LSFMM+BPF), virtual
filesystem (VFS) layer co-maintainer Christian Brauner had a few different
topics he wanted to talk about. Issues on the agenda
included iterating through anonymous mount namespaces, a needed feature
for ID-mapped mounts, the perennial unprivileged mounts topic, potentially
using hazard pointers for file reference counting, and Rust bindings. He
did not expect to get through all of them in the 30 minutes allotted, but
the session did move along pretty quickly to at least
introduce them to the assembled filesystem developers.