dhcp: Continuously try to rebind in t2 (if failed in t1)

The root cause of this issue is big UDP traffic consuming the buffer
so the DHCP doesn't have the bandwitdh to access the Tx buffer.
More info espressif/esp-lwip!217

Ref IDF-4817

src/core/ipv4/dhcp.c

@@ -562,6 +562,11 @@ dhcp_timeout(struct netif *netif)
       dhcp_discover(netif);
     }
   }
+#if ESP_LWIP
+  else if (dhcp->state == DHCP_STATE_REBINDING) {
+    dhcp->t2_rebind_time = 1;
+  }
+#endif /* ESP_LWIP */
 }
 
 /**