libbpf: Fix anonymous type check in CO-RE logic

Use type name for checking whether CO-RE relocation is referring to
anonymous type. Using spec string makes no sense.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220426004511.2691730-2-andrii@kernel.org