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>