BACKPORT: cgroup: avoid copying strings longer than the buffers

cgroup root name and file name have max length limit, we should
avoid copying longer name than that to the name.

tj: minor update to $SUBJ.

Signed-off-by: Ma Shimiao <mashimiao.fnst@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Change-Id: Iff4f30be79184f19d9f3ec253bbab9c4ad91f36c
(cherry picked from commit e7fd37ba12170cc414be8b639dfc2c5f7172fac2)
Bug: 154548692
Signed-off-by: Marco Ballesio <balejs@google.com>
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>