selftests/bpf: Extend map-in-map selftest to detect memory leaks

Add test validating that all inner maps are released properly after skeleton
is destroyed. To ensure determinism, trigger kernel-side synchronize_rcu()
before checking map existence by their IDs.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200729040913.2815687-2-andriin@fb.com