selftests/bpf: add prog detach to flow_dissector test

In case we are not running in a namespace (which we don't do by default),
let's try to detach the bpf program that we use for eth_get_headlen tests.

Fixes: 0905beec ("selftests/bpf: run flow dissector tests in skb-less mode")
Signed-off-by: NStanislav Fomichev <sdf@google.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

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

@@ -264,5 +264,6 @@ void test_flow_dissector(void)
 		CHECK_FLOW_KEYS(tests[i].name, flow_keys, tests[i].keys);
 	}
 
+	bpf_prog_detach(prog_fd, BPF_FLOW_DISSECTOR);
 	bpf_object__close(obj);
 }