In early days of uffd.c return value from uffd_copy was used to count
transferred pages. Since this is not the case anymore we can use 0 as
success.

travis-ci: success for uffd: A new set of improvements
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>