lazy-pages: do not allow background fetch before restore is finished

If we start backroung memory fetch before restore is completely finished,
we may try to write to the memory areas which were not yet remapped to
proper place and are not registered with userfaultfd.
Add synchronization between restore and the lazy-pages so that lazy-pages
will only handle #PFs before all the tasks are restored.

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