cgroup1: don't allow '\n' in renaming

cgroup_mkdir() have restriction on newline usage in names:
$ mkdir $'/sys/fs/cgroup/cpu/test\ntest2'
mkdir: cannot create directory
'/sys/fs/cgroup/cpu/test\ntest2': Invalid argument

But in cgroup1_rename() such check is missed.
This allows us to make /proc/<pid>/cgroup unparsable:
$ mkdir /sys/fs/cgroup/cpu/test
$ mv /sys/fs/cgroup/cpu/test $'/sys/fs/cgroup/cpu/test\ntest2'
$ echo $$ > $'/sys/fs/cgroup/cpu/test\ntest2'
$ cat /proc/self/cgroup
11:pids:/
10:freezer:/
9:hugetlb:/
8:cpuset:/
7:blkio:/user.slice
6:memory:/user.slice
5:net_cls,net_prio:/
4:perf_event:/
3:devices:/user.slice
2:cpu,cpuacct:/test
test2
1:name=systemd:/
0::/

Signed-off-by: Alexander Kuznetsov <[e-mail]>
Reported-by: Andrey Krasichkov <[e-mail]>
Acked-by: Dmitry Yakunin <[e-mail]>
Cc: [e-mail]
Signed-off-by: Tejun Heo <[e-mail]>

kernel/cgroup/cgroup-v1.c

···
 	struct cgroup *cgrp = kn->priv;
 	int ret;
 
+	/* do not accept '\n' to prevent making /proc/<pid>/cgroup unparsable */
+	if (strchr(new_name_str, '\n'))
+		return -EINVAL;
+
 	if (kernfs_type(kn) != KERNFS_DIR)
 		return -ENOTDIR;
 	if (kn->parent != new_parent)