[$] Phyr: a potential scatterlist replacement

The "scatterlist" is a core-kernel data structure used to describe DMA I/Ooperations from the point of view of both the CPU and the peripheraldevice. Over the years, the shortcomings of scatterlists have become moreapparent, but there has not been a viable replacement on the horizon.During a memory-management session at the 2023 Linux Storage, Filesystem, Memory-Managementand BPF Summit, Jason Gunthorpe described a possible alternative, knownalternatively as "phyr", "physr", or "rlist", that might improve onscatterlists for at least some use cases.