[$] Reworking page-table traversal

A system's page tables are organized into a tree that is as many as fivelevels deep. In many ways those levels are all similar, but the kerneltreats them all as being different, with the result that page-tablemanipulations include a fair amount of repetitive code. During thememory-management track of the 2018 Linux Storage, Filesystem, andMemory-Management Summit, Kirill Shutemov proposed reworking how pagetables are maintained. The idea was popular, but the implementation islikely to be tricky.