[$] Cleaning up dying control groups, 2022 edition

Control groups are a useful system-management feature, but they can alsoconsume a lot of resources, especially if they hang around on the systemafter they have been deleted. Roman Gushchin described the problems that can result at the2019 Linux Storage, Filesystem, Memory-management and BPF Summit (LSFMM);he returned during the 2022 LSFMM to revisitthe issue, especially as it relates to the memory controller. Progress hasbeen made, but the problem is not yet solved.