[Virtio-fs] [virtiofsd-rs] MR opened: Set up uid/gid mappings in user_namespace sandbox

[virtiofs-bot at sinrega.org]

When using the namespace sandbox mode with a non-privileged user, set
up 1-to-1 mappings for the current process uid and gid. This allows
both root and a user with the same uid/gid in the VM to access the
files served by us. This scenario fits nicely with the "shared folder"
use case.

Signed-off-by: Sergio Lopez <slp at redhat.com>
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https://gitlab.com/virtio-fs/virtiofsd-rs/-/merge_requests/16