cgroup: Define CGROUP_SUBSYS_COUNT according the configuration

Since we know exactly how many subsystems exists at compile time we are
able to define CGROUP_SUBSYS_COUNT correctly. CGROUP_SUBSYS_COUNT will
be at max 12 (all controllers enabled). Depending on the architecture
we safe either 32 - 12 pointers (80 bytes) or 64 - 12 pointers (416
bytes) per cgroup.

With this change we can also remove the temporary placeholder to avoid
compilation errors.

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Cc: Gao feng <gaofeng@cn.fujitsu.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: John Fastabend <john.r.fastabend@intel.com>
Cc: netdev@vger.kernel.org
Cc: cgroups@vger.kernel.org