[Virtio-fs] [virtiofsd-rs] MR merged: Strip shared directory from mountinfo’s mount points with sandbox=chroot

[virtiofs-bot at sinrega.org]

When using *sandbox=chroot*, `/proc/self/mountinfo` still contains the pre-chroot mount point paths.  To open them, we need to strip the shared directory as a prefix from them.

This series does so by first canonicalizing the shared directory path, and then (when using *sandbox=chroot*) stripping it from every mount point in `/proc/self/mountinfo` that we want to open.

(Reported by German)
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/58