cgroup: Take css_set_lock from cgroup_css_sets_empty()

As indicated in the comment above cgroup_css_sets_empty it needs the
css_set_lock. But neither of the 2 call points have it, so rather then fixing
the callers just take the lock inside cgroup_css_sets_empty().

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Change-Id: If7aea71824f6d0e3f2cc6c1ce236c3ae6be2037b

kernel/cgroup.c

@@ -3977,19 +3977,23 @@ static int cgroup_clear_css_refs(struct cgroup *cgrp)
 	return !failed;
 }
 
-/* checks if all of the css_sets attached to a cgroup have a refcount of 0.
- * Must be called with css_set_lock held */
+/* Checks if all of the css_sets attached to a cgroup have a refcount of 0. */
 static int cgroup_css_sets_empty(struct cgroup *cgrp)
 {
 	struct cg_cgroup_link *link;
+	int retval = 1;
 
+	read_lock(&css_set_lock);
 	list_for_each_entry(link, &cgrp->css_sets, cgrp_link_list) {
 		struct css_set *cg = link->cg;
-		if (atomic_read(&cg->refcount) > 0)
-			return 0;
+		if (atomic_read(&cg->refcount) > 0) {
+			retval = 0;
+			break;
+		}
 	}
+	read_unlock(&css_set_lock);
 
-	return 1;
+	return retval;
 }
 
 static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)