selftests/bpf: fix test_dev_cgroup

The test incorrectly doing
mkdir /mnt/cgroup-test-work-dirtest-bpf-based-device-cgroup
instead of
mkdir /mnt/cgroup-test-work-dir/test-bpf-based-device-cgroup

somehow such mkdir succeeds and new directory appears:
/mnt/cgroup-test-work-dir/cgroup-test-work-dirtest-bpf-based-device-cgroup

Later cleanup via nftw("/mnt/cgroup-test-work-dir", ...);
doesn't walk this directory.
"rmdir /mnt/cgroup-test-work-dir" succeeds, but bpf program and
dangling cgroup stays in memory.
That's a separate issue on a cgroup side.
For now fix the test.

Fixes: 37f1ba09

 ("selftests/bpf: add a test for device cgroup controller")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>