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 <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Acked-by: Hao Luo <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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