selftests/bpf: Add test for cgroup iterator on a dead cgroup

selftests/bpf: Add test for cgroup iterator on a dead cgroup

The test closes both iterator link fd and cgroup fd, and removes the
cgroup file to make a dead cgroup before reading from cgroup iterator.
It also uses kern_sync_rcu() and usleep() to wait for the release of
start cgroup. If the start cgroup is not pinned by cgroup iterator,
reading from iterator fd will trigger use-after-free.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Hao Luo <haoluo@google.com>
Link: https://lore.kernel.org/bpf/20221121073440.1828292-4-houtao@huaweicloud.com