lazy-pages: treat ENOMEM returned by UFFDIO_UNREGISTER as task exit

The UFFDIO_UNREGISTER returns ENOMEM only when it is unable to get
mm_struct. We can assume in such case that the task is exited.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>