selftests/bpf: validate nulled-out struct_ops program is handled properly

selftests/bpf: validate nulled-out struct_ops program is handled properly

Add a selftests validating that it's possible to have some struct_ops
callback set declaratively, then disable it (by setting to NULL)
programmatically. Libbpf should detect that such program should
not be loaded. Otherwise, it will unnecessarily fail the loading
when the host kernel does not have the type information.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240428030954.3918764-2-andrii@kernel.org
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>