Merge branch 'libbpf: further struct_ops fixes and improvements'

Andrii Nakryiko says:

====================
Fix yet another case of mishandling SEC("struct_ops") programs that were
nulled out programmatically through BPF skeleton by the user.

While at it, add some improvements around detecting and reporting errors,
specifically a common case of declaring SEC("struct_ops") program, but
forgetting to actually make use of it by setting it as a callback
implementation in SEC(".struct_ops") variable (i.e., map) declaration.

A bunch of new selftests are added as well.
====================

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>