[$] Cleaning up dying control groups, 2022 edition

Control groups are a useful system-management feature, but they can also
consume a lot of resources, especially if they hang around on the system
after they have been deleted. Roman Gushchin described the problems that can result at the
2019 Linux Storage, Filesystem, Memory-management and BPF Summit (LSFMM);
he returned during the 2022 LSFMM to revisit
the issue, especially as it relates to the memory controller. Progress has
been made, but the problem is not yet solved.