add mangle table in the iptable flush task (#5672)

When kube-router is used as cni, rules might be added to the mangle table
to support external IPs. Therefore, mangle table should be flushed during
reset as well.

roles/reset/tasks/main.yml

@@ -157,6 +157,7 @@
   with_items:
     - filter
     - nat
+    - mangle
   when: flush_iptables|bool
   tags:
     - iptables