Use constant instead of magic number

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

apps/files/lib/Service/OwnershipTransferService.php

@@ -194,7 +194,7 @@ class OwnershipTransferService {
 		$output->writeln('Validating quota');
 		$size = $view->getFileInfo($sourcePath, false)->getSize(false);
 		$freeSpace = $view->free_space($destinationUid . '/files/');
-		if ($size > $freeSpace && $freeSpace !== -2) {
+		if ($size > $freeSpace && $freeSpace !== FileInfo::SPACE_UNKNOWN) {
 			$output->writeln('<error>Target user does not have enough free space available.</error>');
 			throw new \Exception('Execution terminated.');
 		}