[$] Blocking userfaultfd() kernel-fault handling

The userfaultfd()system call is a bit of a strange beast; it allows user space to takeresponsibility for the handling of page faults, which is normally aquintessential kernel task. It is thus perhaps not surprising that it hasturned out to have some utility for those who would attack the kernel'ssecurity as well. A recent patchset from Daniel Colascione is small, but it makes a significant changethat can help block at least one sort of attack usinguserfaultfd().