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 v5.4.134 by commit c17363ccd620c1a57ede00d5c777f0b8624debe6, it was introduced in version v5.1 by commit 8d2451f4994fa60a57617282bab91b98266a00b1. For more details please see the references link.