selftests/bpf: Add tests for attaching bpf_link to netns

Extend the existing test case for flow dissector attaching to cover:

 - link creation,
 - link updates,
 - link info querying,
 - mixing links with direct prog attachment.

Signed-off-by: Jakub Sitnicki <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: jsitnicki