[$] Fleshing out memory descriptors

One of the long-term goals of the folio conversion in the kernel'smemory-management subsystem is the replacement of the pagestructure, which describes a page of physical memory, with an eight-byte"memory descriptor". This change would reduce the overhead of trackingphysical memory, increase type safety, and make memory management moreflexible. Thus far, though, details on what the memory-descriptor futurewill look like have been relatively scarce. At the 2024 Linux Storage,Filesystem, Memory-Management and BPF Summit, Matthew Wilcox led adiscussion to try to fill in the picture somewhat.