selftests/bpf: add another struct_ops callback use case test

Add a test which tests the case that was just fixed. Kernel has full
type information about callback, but user explicitly nulls out the
reference to declaratively set BPF program reference.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240507001335.1445325-4-andrii@kernel.org


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>