Merge branch 'libbpf: deprecate legacy BPF map definitions'

Andrii Nakryiko says:

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

Officially deprecate legacy BPF map definitions in libbpf. They've been slated
for deprecation for a while in favor of more powerful BTF-defined map
definitions and this patch set adds warnings and a way to enforce this in
libbpf through LIBBPF_STRICT_MAP_DEFINITIONS strict mode flag.

Selftests are fixed up and updated, BPF documentation is updated, bpftool's
strict mode usage is adjusted to avoid breaking users unnecessarily.

v1->v2:
  - replace missed bpf_map_def case in Documentation/bpf/btf.rst (Alexei).
====================

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