Revoke sharable permission on automatic SMB shares

Shares authenticated with user credentials (aka not hard coded) cannot be
accessed by other users, breaking sharing. This change completely revokes
sharing for such shares

apps/files_external/lib/smb_auto.php

@@ -43,4 +43,8 @@ class SMB_Auto extends \OC\Files\Storage\SMB{
 	public static function login( $params ) {
 		\OC::$session->set('smb-credentials', $params);
 	}
+
+	public function isSharable($path) {
+		return false;
+	}
 }