selftests/bpf: add test for BPF flow dissector in the root namespace

Make sure non-root namespaces get an error if root flow dissector is
attached.

Cc: Petar Penkov <ppenkov@google.com>
Acked-by: Song Liu <songliubraving@fb.com>

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>