lazy-pages: uffd_{copy, zeropage}: rework error handling

Improve code readability and take into account that even if
uffdio_{copy,zeropage} has failed, some pages had been updated.

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