Persistent workload throttling via uncleared cgroup delay state

Denial of service in Linux 4.19 blk-iolatency allows local users to persistently throttle cgroup workloads via stale delay state. In block/blk-iolatency.c, iolatency_pd_offline() clears delay only when a nonzero latency target becomes zero, leaving delay and congestion accounting set for throttled peers that never had a target. The condition requires sibling latency pressure followed by policy-data teardown; affected cgroups and descendants retain reduced readahead and anonymous-memory allocation throttling for the cgroup's lifetime, without a reported crash.