selftests/bpf: test case for register_bpf_struct_ops().

Create a new struct_ops type called bpf_testmod_ops within the bpf_testmod
module. When a struct_ops object is registered, the bpf_testmod module will
invoke test_2 from the module.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20240119225005.668602-15-thinker.li@gmail.com


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