[$] Sunsetting buffer heads

The buffer head is a kernel data structure that dates back to the firstLinux release; for much of the time since then, kernel developers have beenhoping to get rid of it. Hannes Reineckestarted a plenary session at the 2023 Linux Storage, Filesystem, Memory-Managementand BPF Summit by saying that everybody agrees that buffer heads are abad idea, but there is less agreement on how to take them out of thekernel. The core functionality they provide - facilitating sector-size I/Ooperations to a block device underlying a filesystem - must be providedsomehow.