selftests/bpf: Add selftest for bpf_task_under_cgroup() in sleepable prog

The result is as follows:

$ tools/testing/selftests/bpf/test_progs --name=task_under_cgroup
#237 task_under_cgroup:OK
Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Without the previous patch, there will be RCU warnings in dmesg when
CONFIG_PROVE_RCU is enabled. While with the previous patch, there will
be no warnings.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20231007135945.4306-2-laoar.shao@gmail.com

selftests/bpf: Add testcase for bpf_task_under_cgroup

test_progs:
Tests new kfunc bpf_task_under_cgroup().

The bpf program saves the new task's pid within a given cgroup to
the remote_pid, which is convenient for the user-mode program to
verify the test correctness.

The user-mode program creates its own mount namespace, and mounts the
cgroupsv2 hierarchy in there, call the fork syscall, then check if
remote_pid and local_pid are unequal.

Signed-off-by: Feng Zhou <zhoufeng.zf@bytedance.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20230506031545.35991-3-zhoufeng.zf@bytedance.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>