bpf: selftests: Test fentry tracing a struct_ops program

This patch tests attaching an fentry prog to a struct_ops prog.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220330011502.2985292-1-kafai@fb.com