[$] Write-protect for userfaultfd()

The userfaultfd()system call allows one process to handle page faults for another - in userspace. Its original use case was to support transparent containermigration, but other uses have developed over the years. At the 2019 LinuxStorage, Filesystem, and Memory-Management Summit, Andrea Arcangelidescribed a scheme to add write-protectionsupport to userfaultfd(). After a year of lost time fightingspeculative-execution problems, Arcangeli is about ready to move this featureinto the mainline.