lazy-pages: make uffd_{copy,zero} return 0 on success

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>

criu/uffd.c

@@ -544,7 +544,7 @@ static int uffd_copy(struct lazy_pages_info *lpi, __u64 address, int nr_pages)
 
 	lpi->copied_pages += nr_pages;
 
-	return uffdio_copy.copy;
+	return 0;
 }
 
 static int uffd_zero(struct lazy_pages_info *lpi, __u64 address, int nr_pages)
@@ -566,7 +566,7 @@ static int uffd_zero(struct lazy_pages_info *lpi, __u64 address, int nr_pages)
 		return -1;
 	}
 
-	return len;
+	return 0;
 }
 
 static int uffd_handle_pages(struct lazy_pages_info *lpi, __u64 address, int nr)