kselftest/cgroup: fix unexpected testing failure on test_core

The cgroup testing relys on the root cgroup's subtree_control setting,
If the 'memory' controller isn't set, some test cases will be failed
as following:

$sudo  ./test_core
not ok 1 test_cgcore_internal_process_constraint
ok 2 test_cgcore_top_down_constraint_enable
not ok 3 test_cgcore_top_down_constraint_disable
...

To correct this unexpected failure, this patch write the 'memory' to
subtree_control of root to get a right result.
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

@@ -376,6 +376,11 @@ int main(int argc, char *argv[])
 
 	if (cg_find_unified_root(root, sizeof(root)))
 		ksft_exit_skip("cgroup v2 isn't mounted\n");
+
+	if (cg_read_strstr(root, "cgroup.subtree_control", "memory"))
+		if (cg_write(root, "cgroup.subtree_control", "+memory"))
+			ksft_exit_skip("Failed to set memory controller\n");
+
 	for (i = 0; i < ARRAY_SIZE(tests); i++) {
 		switch (tests[i].fn(root)) {
 		case KSFT_PASS: