selftests/bpf: Cleanup bpf qdisc selftests

Some cleanups:
- Remove unnecessary kfuncs declaration
- Use _ns in the test name to run tests in a separate net namespace
- Call skeleton __attach() instead of bpf_map__attach_struct_ops() to
  simplify tests.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>