cgroup: use for_each_subsys() instead of for_each_root_subsys() in cgroup_populate/clear_dir()

rebind_subsystems() will be updated to handle file creations and
removals with proper error handling and to do that will need to
perform file operations before actually adding the subsystem to the
hierarchy.

To enable such usage, update cgroup_populate/clear_dir() to use
for_each_subsys() instead of for_each_root_subsys() so that they
operate on all subsystems specified by @subsys_mask whether that
subsystem is currently bound to the hierarchy or not.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

kernel/cgroup.c

@@ -965,10 +965,12 @@ static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
 static void cgroup_clear_dir(struct cgroup *cgrp, unsigned long subsys_mask)
 {
 	struct cgroup_subsys *ss;
+	int i;
 
-	for_each_root_subsys(cgrp->root, ss) {
+	for_each_subsys(ss, i) {
 		struct cftype_set *set;
-		if (!test_bit(ss->subsys_id, &subsys_mask))
+
+		if (!test_bit(i, &subsys_mask))
 			continue;
 		list_for_each_entry(set, &ss->cftsets, node)
 			cgroup_addrm_files(cgrp, NULL, set->cfts, false);
@@ -4177,12 +4179,13 @@ static struct cftype cgroup_base_files[] = {
 static int cgroup_populate_dir(struct cgroup *cgrp, unsigned long subsys_mask)
 {
 	struct cgroup_subsys *ss;
-	int ret = 0;
+	int i, ret = 0;
 
 	/* process cftsets of each subsystem */
-	for_each_root_subsys(cgrp->root, ss) {
+	for_each_subsys(ss, i) {
 		struct cftype_set *set;
-		if (!test_bit(ss->subsys_id, &subsys_mask))
+
+		if (!test_bit(i, &subsys_mask))
 			continue;
 
 		list_for_each_entry(set, &ss->cftsets, node) {