When changing the share owner, downgrade permissions where needed

Signed-off-by: Julius Härtl <jus@bitgrid.net>

apps/files_sharing/lib/Updater.php

@@ -87,6 +87,7 @@ class Updater {
 				continue;
 			}
 			$share->setShareOwner($newOwner);
+			$share->setPermissions($share->getPermissions() & $dstMount->getShare()->getPermissions());
 			$shareManager->updateShare($share);
 		}
 	}