Glue code to the userfaultfd kernel implementation.
Querying feature support, createing file descriptor, feature control,
memory region registration, IOCTLs on registered registered regions.

Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210129101407.103458-3-andrey.gruzdev@virtuozzo.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  Fixed up range.start casting for 32bit