output-json-file: use size instead of FileSize

FileSize is not returning the actual value when file store is not
used.

src/output-json-file.c

@@ -171,7 +171,7 @@ static void FileWriteJsonRecord(JsonFileLogThread *aft, const Packet *p, const F
     if (ff->flags & FILE_STORED) {
         json_object_set_new(fjs, "file_id", json_integer(ff->file_id));
     }
-    json_object_set_new(fjs, "size", json_integer(FileSize(ff)));
+    json_object_set_new(fjs, "size", json_integer(ff->size));
     json_object_set_new(fjs, "tx_id", json_integer(ff->txid));
 
     /* originally just 'file', but due to bug 1127 naming it fileinfo */