kselftest/cgroup: fix incorrect test_core skip

The test_core will skip the
test_cgcore_no_internal_process_constraint_on_threads test case if the
'cpu' controller missing in root's subtree_control. In fact we need to
set the 'cpu' in subtree_control, to make the testing meaningful.

./test_core
...
ok 4 # skip test_cgcore_no_internal_process_constraint_on_threads
...
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Claudio Zumbo <claudioz@fb.com>
Cc: Claudio <claudiozumbo@gmail.com>
Cc: linux-kselftest@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Roman Gushchin <guro@fb.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/cgroup/test_core.c

@@ -198,7 +198,7 @@ static int test_cgcore_no_internal_process_constraint_on_threads(const char *roo
 	char *parent = NULL, *child = NULL;
 
 	if (cg_read_strstr(root, "cgroup.controllers", "cpu") ||
-	    cg_read_strstr(root, "cgroup.subtree_control", "cpu")) {
+	    cg_write(root, "cgroup.subtree_control", "+cpu")) {
 		ret = KSFT_SKIP;
 		goto cleanup;
 	}