[$] The path to deprecating SPARSEMEM

The term "memory model" is used in a couple of ways within the kernel.Perhaps the more obscure meaning is the memory-management subsystem's viewof how physical memory is organized on a given system. A properrepresentation of physical memory will be more efficient in terms of memoryand CPU use. Since hardware comes in numerous variations, the kernelsupports a number of memory models to match; see this article for details. At the 2024 Linux Storage,Filesystem, Memory-Management and BPF Summit, Oscar Salvador,presenting remotely, made the case for removing one of those models.