Merge branch 'libbpf: add netfilter link attach helper'

Florian Westphal says:

====================
v4: address comment from Daniel Xu:
  - use human-readable test names in 2/2

v3: address comments from Andrii:
  - prune verbose error message in 1/2
  - use bpf_link_create internally in 1/2
  - use subtests in patch 2/2

When initial netfilter bpf program type support got added one
suggestion was to extend libbpf with a helper to ease attachment
of nf programs to the hook locations.

Add such a helper and a demo test case that attaches a dummy
program to various combinations.

I tested that the selftest fails when changing the expected
outcome (i.e., set 'success' when it should fail and v.v.).
====================

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>