Factor out BTF map definition logic into stand-alone routine for easier reuse
for map-in-map case.
Signed-off-by: NAndrii Nakryiko <andriin@fb.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200429002739.48006-2-andriin@fb.com