cgroup: move cgroup_clear_directory() call out of cgroup_populate_dir()

cgroup_populate_dir() currently clears all files and then repopulate
the directory; however, the clearing part is only useful when it's
called from cgroup_remount().  Relocate the invocation to
cgroup_remount().

This is to prepare for further cgroup file handling updates.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizf@cn.fujitsu.com>

kernel/cgroup.c

@@ -1313,7 +1313,8 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
 		goto out_unlock;
 	}
 
-	/* (re)populate subsystem files */
+	/* clear out any existing files and repopulate subsystem files */
+	cgroup_clear_directory(cgrp->dentry);
 	cgroup_populate_dir(cgrp);
 
 	if (opts.release_agent)
@@ -3644,9 +3645,6 @@ static int cgroup_populate_dir(struct cgroup *cgrp)
 	int err;
 	struct cgroup_subsys *ss;
 
-	/* First clear out any existing files */
-	cgroup_clear_directory(cgrp->dentry);
-
 	err = cgroup_add_files(cgrp, NULL, files, ARRAY_SIZE(files));
 	if (err < 0)
 		return err;