selftests/bpf: Migrate struct_ops_assoc test to KF_IMPLICIT_ARGS

selftests/bpf: Migrate struct_ops_assoc test to KF_IMPLICIT_ARGS

A test kfunc named bpf_kfunc_multi_st_ops_test_1_impl() is a user of
__prog suffix. Subsequent patch removes __prog support in favor of
KF_IMPLICIT_ARGS, so migrate this kfunc to use implicit argument.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260120222638.3976562-12-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>