namespace: Load sidecar verity settings in apply_mount_namespace()

namespace: Load sidecar verity settings in apply_mount_namespace()

Let's reduce the argument count of setup_namespace() a bit by loading
the sidecar verity settings in apply_mount_namespace(). This will also
make it possible to pass file descriptors to the root image/directory
into setup_namespace() as before this wasn't possible because the
verity settings logic looks for sidecar files next to the
root image which requires the path to be available.