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

tools/testing/selftests/bpf/prog_tests/xdp_metadata.c

@@ -205,9 +205,8 @@ static void complete_tx(struct xsk *xsk)
 	if (ASSERT_EQ(xsk_ring_cons__peek(&xsk->comp, 1, &idx), 1, "xsk_ring_cons__peek")) {
 		addr = *xsk_ring_cons__comp_addr(&xsk->comp, idx);
 
-		printf("%p: refill idx=%u addr=%llx\n", xsk, idx, addr);
-		*xsk_ring_prod__fill_addr(&xsk->fill, idx) = addr;
-		xsk_ring_prod__submit(&xsk->fill, 1);
+		printf("%p: complete tx idx=%u addr=%llx\n", xsk, idx, addr);
+		xsk_ring_cons__release(&xsk->comp, 1);
 	}
 }