Pickup update libbtf that supports anonymous inner maps

[Alan-Jowett]

This map definition fails verification because libbtf doesn't return the descriptor for maps that aren't associated with variables in the .maps section (like an anonymous inner map).

__attribute__((section(".maps"), used))
struct {
    __uint(type, BPF_MAP_TYPE_ARRAY_OF_MAPS);
    __uint(max_entries, 1);
    __type(key, uint32_t);
    __array(values, struct
    {
        __uint(type, BPF_MAP_TYPE_ARRAY);
        __type(key, uint32_t);
        __type(value, uint32_t);
        __uint(max_entries, 1);
    });
} outer_map;

Pickup updated libbtf and ebpf-samples.
Add test to check verification of anonymous map-in-map.

[dthaler]

@Alan-Jowett please rebase, which should fix the test failures in CI/CD

[coveralls]

coverage: 89.527% (+0.001%) from 89.526%
when pulling 4e817a6 on Alan-Jowett:update_libbtf
into 7311c35 on vbpf:main.