cgroup: make cgroup_is_threaded() and cgroup_is_thread_root() static

They're only called inside cgroup.c. Make them static.
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup-internal.h

@@ -220,8 +220,6 @@ static inline void get_css_set(struct css_set *cset)
 
 bool cgroup_ssid_enabled(int ssid);
 bool cgroup_on_dfl(const struct cgroup *cgrp);
-bool cgroup_is_thread_root(struct cgroup *cgrp);
-bool cgroup_is_threaded(struct cgroup *cgrp);
 
 struct cgroup_root *cgroup_root_from_kf(struct kernfs_root *kf_root);
 struct cgroup *task_cgroup_from_root(struct task_struct *task,

kernel/cgroup/cgroup.c

@@ -357,7 +357,7 @@ static bool cgroup_has_tasks(struct cgroup *cgrp)
 	return cgrp->nr_populated_csets;
 }
 
-bool cgroup_is_threaded(struct cgroup *cgrp)
+static bool cgroup_is_threaded(struct cgroup *cgrp)
 {
 	return cgrp->dom_cgrp != cgrp;
 }
@@ -396,7 +396,7 @@ static bool cgroup_can_be_thread_root(struct cgroup *cgrp)
 }
 
 /* is @cgrp root of a threaded subtree? */
-bool cgroup_is_thread_root(struct cgroup *cgrp)
+static bool cgroup_is_thread_root(struct cgroup *cgrp)
 {
 	/* thread root should be a domain */
 	if (cgroup_is_threaded(cgrp))