lazy-pages: consolidate error handling for uffdio_copy and uffdio_zeropage

The errors set by both uffdio_copy and uffdio_zeropage are the same and
require the same handling. Let's use a helper function to handle the errors
returned by uffdio_copy and uffdio_zero.

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