[$] A last-minute MMU notifier change

One does not normally expect to see significant changes to an importantinternal memory-management mechanism in the time between the "rc7 prepatchand the final release for a development cycle, but that is exactly whathappened just before 4.13 was released. A regression involving thememory-management unit (MMU) notifier mechanism briefly threatened todelay this release, but a last-minute scramble kept 4.13 onschedule and also resulted in a cleanup of that mechanism.This seems like a good time to look at a mechanism that LinusTorvalds called "a badly designed mistake" and how it was madeto be a bit less mistaken.