core: use DUAL_TIMESTAMP_NULL to reset kernel_timestamp

instead of direct reset of kernel_timestamp fields.

src/core/main.c

@@ -1435,9 +1435,7 @@ int main(int argc, char *argv[]) {
 
                 /* clear the kernel timestamp,
                  * because we are in a container */
-                kernel_timestamp.monotonic = 0ULL;
-                kernel_timestamp.realtime = 0ULL;
-
+                kernel_timestamp = DUAL_TIMESTAMP_NULL;
         } else {
                 /* Running as user instance */
                 arg_running_as = MANAGER_USER;