Pass 0 as target_fd when attaching and detaching flow dissector.
Additionally, pass the expected program when detaching.

Fixes: 1f043f87 ("selftests/bpf: Add tests for attaching bpf_link to netns")
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200629095630.7933-6-lmb@cloudflare.com