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: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>