Filesystem mounts in user namespaces — one year later

The primary target for now remains fuse.

We may be able to support other filesystems eventually but fuse should be supportable now.
Do you have any fuzz testing with respect to fuse?

The work of handling the odd cases was performed with respect to the vfs and not fuse as the necessary changes are cleaner and more obvious that way.

Fuse presents a very interesting case as it allows isolating the filesystem code in userspace while still allowing that code to be used by all programs through the vfs.

Fuse can support all of the popular filesystems today, and as such provides a safer alternative to mounting filesystems on usb sticks. The user namespace mount aspect of this just this all more usable.