lazy-pages: handle_exit: set uffd to negative value instead of 0

The only use for the userfault file descriptor after the process exited is
for debug logs. Using negative value instead of 0 makes logs more readable.
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/uffd.c

@@ -789,7 +789,7 @@ static int handle_exit(struct lazy_pages_info *lpi)
 		return -1;
 	free_iovs(lpi);
 	close(lpi->lpfd.fd);
-	lpi->lpfd.fd = 0;
+	lpi->lpfd.fd = -lpi->lpfd.fd;
 	lpi->exited = true;
 
 	/* keep it for tracking in-flight requests and for the summary */