[$] Preventing stack guard-page hopping

Normally, the -rc6 kernel testing release is not the place where one wouldexpect to find a 900-line memory-management change. As it happens, though,such a change was quietly merged immediately prior to the 4.12-rc6 release; indeed, it may have been thereal reason behind 4.12-rc6 coming out some hours later than would havebeen expected. This change is important, though, in that it addresses anewly publicized security threat that, it seems, is being activelyexploited.