[Virtio-fs] [virtiofsd-rs] MR opened: Correctly open mountinfo with sandbox=none

[virtiofs-bot at sinrega.org]

We need to open `/proc/self/mountinfo` with `O_RDONLY`, because we want to read from it.  Instead, we open it with `O_PATH` (when using sandbox=none), though, which is wrong and will later prevent us from creating openable file handles.

Fix it.

(Reported by German)
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https://gitlab.com/virtio-fs/virtiofsd-rs/-/merge_requests/52