userfaultfd: add /dev/userfaultfd for fine grained access control
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2158706
This patchset backports the /dev/userfaultfd device. It is another way to create an userfaultfd file descriptor, and it is preferred in some restricted environments (e.g. where the userfaultfd() syscall is forbidden).
Signed-off-by: Peter Xu peterx@redhat.com