selftests/bpf: Don't refill on completion in xdp_metadata

We only need to consume TX completion instead of refilling 'fill' ring.
It's currently not an issue because we never RX more than 8 packets.

Fixes: e2a46d54

 ("selftests/bpf: Verify xdp_metadata xdp->af_xdp path")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230201233640.367646-1-sdf@google.com