selftests/bpf: Unmount the cgroup2 work directory

test_progs -t bind_perm,bpf_obj_pinning/mounted-str-rel fails when
the selftests directory is mounted under /mnt, which is a reasonable
thing to do when sharing the selftests residing on the host with a
virtual machine, e.g., using 9p.

The reason is that cgroup2 is mounted at /mnt and not unmounted,
causing subsequent tests that need to access the selftests directory
to fail.

Fix by unmounting it. The kernel maintains a mount stack, so this
reveals what was mounted there before. Introduce cgroup_workdir_mounted
in order to maintain idempotency. Make it thread-local in order to
support test_progs -j.

Signed-off-by: Ilya Leoshkevich <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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