Merge branch 'bpf: bpf link iterator'

Dmitrii Dolgov says:

====================

Bpf links seem to be one of the important structures for which no
iterator is provided. Such iterator could be useful in those cases when
generic 'task/file' is not suitable or better performance is needed.

The implementation is mostly copied from prog iterator. This time tests were
executed, although I still had to exclude test_bpf_nf (failed to find BTF info
for global/extern symbol 'bpf_skb_ct_lookup') -- since it's unrelated, I hope
it's a minor issue.

Per suggestion from the previous discussion, there is a new patch for
converting CHECK to corresponding ASSERT_* macro. Such replacement is done only
if the final result would be the same, e.g. CHECK with important-looking custom
formatting strings are still in place -- from what I understand ASSERT_*
doesn't allow to specify such format.

The third small patch fixes what looks like a copy-paste error in the condition
checking.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>