[Virtio-fs] [virtiofsd] MR merged: sandbox: Move parent process code inside sandbox

[virtiofs-bot at sinrega.org]

Only the namespace sandbox forks and the parent process
just waits for the child termination. This commit moves
the waiting code to a helper function, now the parent
waits inside the enter_namespace(). This will ease the
implementation of user namespace uid/gis mapping.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/118