UVI-2021-1001213 cgroup: verify that source is a string

cgroup: verify that source is a string This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.13.4 by commit a41573667b39152176f6b08d10b4deb171e541c...