selftests/bpf: Test case for lacking CFI stub functions.

Ensure struct_ops rejects the registration of struct_ops types without
proper CFI stub functions.

bpf_test_no_cfi.ko is a module that attempts to register a struct_ops type
called "bpf_test_no_cfi_ops" with cfi_stubs of NULL and non-NULL value.
The NULL one should fail, and the non-NULL one should succeed. The module
can only be loaded successfully if these registrations yield the expected
results.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20240222021105.1180475-3-thinker.li@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>