selftests/bpf: test eager BPF ringbuf size adjustment logic

Add test validating that libbpf adjusts (and reflects adjusted) ringbuf
size early, before bpf_object is loaded. Also make sure we can't
successfully resize ringbuf map after bpf_object is loaded.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220715230952.2219271-2-andrii@kernel.org


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