libbpf: Free up resources used by inner map definition

It's not enough to just free(map->inner_map), as inner_map itself can
have extra memory allocated, like map name.

Fixes: 646f02ff ("libbpf: Add BTF-defined map-in-map support")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Hengqi Chen <hengqi.chen@gmail.com>
Link: https://lore.kernel.org/bpf/20211107165521.9240-3-andrii@kernel.org