selftests/bpf: ringbuf_multi: Test bpf_map__set_inner_map_fd

Test map__set_inner_map_fd() interaction with map-in-map
initialization. Use hashmap of maps just to make it different to
existing array of maps.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210408061310.95877-9-yauheni.kaliuta@redhat.com