[$] Stabilizing per-VMA locking

The kernel-development process routinely absorbs large changes tofundamental subsystems and still produces stable releases every nine or tenweeks. On occasion, though, the development community's luck runs out.The per-VMA locking work that went into the6.4 release is a case in point; it looked like a well-tested change thatimproved page-fault scalability. There turned out to be a few demonshiding in that code, though, that made life difficult for early adopters ofthe 6.4 kernel.