selftests/bpf: Add multi_st_ops that supports multiple instances

Current struct_ops in bpf_testmod only support attaching single instance.
Add multi_st_ops that supports multiple instances. The struct_ops uses map
id as the struct_ops id and will reject attachment with an existing id.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250806162540.681679-3-ameryhung@gmail.com