selftests/bpf: Add retval test for bool and errno LSM cgroup hooks

selftests/bpf: Add retval test for bool and errno LSM cgroup hooks

Add test to check the return value when a BPF program exits with 0 for
a boolean and an errno LSM hook.

For each hook, two BPF programs are attached. The first program returns
0 without calling bpf_set_retval() to exercise the return value translation
logic, while the second program reads the retval via bpf_get_retval().

Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Link: https://lore.kernel.org/r/20260610201724.733943-3-xukuohai@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>