coccinelle: fix the log-json rule

As it generated very questionable results.

coccinelle/log-json.cocci

@@ -3,7 +3,6 @@
 expression e, v, flags;
 expression list args;
 @@
-+ return
 - json_log(v, flags, 0, args);
-+ json_log(v, flags, SYNTHETIC_ERRNO(e), args);
 - return -e;
++ return json_log(v, flags, SYNTHETIC_ERRNO(e), args);