cgroup: use do_each_subsys_mask() where applicable

There are several places in cgroup_subtree_control_write() which can
use do_each_subsys_mask() instead of manual mask testing.  Use it.

No functional changes.

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