CVE-2021-47209 sched/fair: Prevent dead task groups from regaining cfs_rq's

In the Linux kernel, the following vulnerability has been resolved: sched/fair: Prevent dead task groups from regaining cfsrq's Kevin is reporting crashes which point to a use-after-free of a cfsrq in updateblockedaverages. Initial debugging revealed that we've live cfsrq's onlist=1 in an about t...