cgroup: make cgroup subsystem masks u16

After the recent do_each_subsys_mask() conversion, there's no reason
to use ulong for subsystem masks.  We'll be adding more subsystem
masks to persistent data structures, let's reduce its size to u16
which should be enough for now and the foreseeable future.

This doesn't create any noticeable behavior differences.

v2: Johannes spotted that the initial patch missed cgroup_no_v1_mask.
    Converted.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>

include/linux/cgroup-defs.h

@@ -257,8 +257,8 @@ struct cgroup {
 	 * one which may have more subsystems enabled.  Controller knobs
 	 * are made available iff it's enabled in ->subtree_control.
 	 */
-	unsigned long subtree_control;
-	unsigned long subtree_ss_mask;
+	u16 subtree_control;
+	u16 subtree_ss_mask;
 
 	/* Private pointers for each registered subsystem */
 	struct cgroup_subsys_state __rcu *subsys[CGROUP_SUBSYS_COUNT];