elf: copy InnerMap spec
elf: copy InnerMap spec

Previously, MapInMap.InnerMap just pointed at an existing MapSpec. Changing
InnerMap would end up also changing the template map. This is not very intuitive,
so fix this by always copying the template.

cilium/ebpf,cilium/ebpf,nathanjsweet/ebpf,nathanjsweet/ebpf