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>