[$] Remote memory control-group charging

Memory control groups exist to track and limit the amount of memory used by
sets of processes. Normally, one would not expect that memory used by one
group would be charged to another but, as Shakeel Butt described in a
memory-management track session at the 2019 Linux Storage, Filesystem, and
Memory-Management Summit, that does happen in a number of different
situations. It's often a problem, but occasionally it's also a useful
feature.