libbpf: Add bpf_map__inner_map API

The API gives access to inner map for map in map types (array or
hash of map). It will be used to dynamically set max_entries in it.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210408061310.95877-7-yauheni.kaliuta@redhat.com