cgroup: use cgroup_addrm_files() in cgroup_clear_directory()

cgroup_clear_directory() incorrectly invokes cgroup_rm_file() on each
cftset of the target subsystems, which only removes the first file of
each set.  This leaves dangling files after subsystems are removed
from a cgroup root via remount.

Use cgroup_addrm_files() to remove all files of target subsystems.

tj: Move cgroup_addrm_files() prototype decl upwards next to other
    global declarations.  Commit message updated.
Signed-off-by: NGao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -246,6 +246,8 @@ static DEFINE_SPINLOCK(hierarchy_id_lock);
 static int need_forkexit_callback __read_mostly;
 
 static int cgroup_destroy_locked(struct cgroup *cgrp);
+static int cgroup_addrm_files(struct cgroup *cgrp, struct cgroup_subsys *subsys,
+			      struct cftype cfts[], bool is_add);
 
 #ifdef CONFIG_PROVE_LOCKING
 int cgroup_lock_is_held(void)
@@ -964,7 +966,7 @@ static void cgroup_clear_directory(struct dentry *dir, bool base_files,
 		if (!test_bit(ss->subsys_id, &subsys_mask))
 			continue;
 		list_for_each_entry(set, &ss->cftsets, node)
-			cgroup_rm_file(cgrp, set->cfts);
+			cgroup_addrm_files(cgrp, NULL, set->cfts, false);
 	}
 	if (base_files) {
 		while (!list_empty(&cgrp->files))