libbpf: improve old BPF skeleton handling for map auto-attach

Improve how we handle old BPF skeletons when it comes to BPF map
auto-attachment. Emit one warn-level message per each struct_ops map
that could have been auto-attached, if user provided recent enough BPF
skeleton version. Don't spam log if there are no relevant struct_ops
maps, though.

This should help users realize that they probably need to regenerate BPF
skeleton header with more recent bpftool/libbpf-cargo (or whatever other
means of BPF skeleton generation).

Signed-off-by: Andrii Nakryiko <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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