cgroup: make cgroup.threads delegatable

Make cgroup.threads file delegatable.
The behavior of cgroup.threads should follow the behavior of cgroup.procs.

Signed-off-by: Roman Gushchin <guro@fb.com>
Discovered-by: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup.c

@@ -4447,6 +4447,7 @@ static struct cftype cgroup_base_files[] = {
 	},
 	{
 		.name = "cgroup.threads",
+		.flags = CFTYPE_NS_DELEGATABLE,
 		.release = cgroup_procs_release,
 		.seq_start = cgroup_threads_start,
 		.seq_next = cgroup_procs_next,