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: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/test_dev_cgroup.c

@@ -21,7 +21,7 @@
 
 #define DEV_CGROUP_PROG "./dev_cgroup.o"
 
-#define TEST_CGROUP "test-bpf-based-device-cgroup/"
+#define TEST_CGROUP "/test-bpf-based-device-cgroup/"
 
 int main(int argc, char **argv)
 {