Fix arp_responder.py failed to start in many tests

Summary:
Fixes arp_responder.py failed to start due to receiving unknown ethernet type packets

What is the motivation for this PR?
When ARP responder received malformed packet, it cannot handle it and just crashed. This caused a lot of testcase failures.

How did you do it?
Add more logic to check the received packet.

How did you verify/test it?
Run test_acl.py::TestBasicACL and got 100% pass

Author: wsycqyz

tests/scripts/arp_responder.py

@@ -99,11 +99,23 @@ def __init__(self, ip_sets):
 
     def action(self, interface):
         data = interface.recv()
-
-        if len(data) <= self.ARP_PKT_LEN:
-            return self.reply_to_arp(data, interface)
-        elif len(data) <= self.NDP_PKT_LEN:
-            return self.reply_to_ndp(data, interface)
+        eth_type = struct.unpack('!H', data[12:14])[0]
+
+        if eth_type == 0x0806:  # ARP
+            if len(data) <= self.ARP_PKT_LEN:
+                return self.reply_to_arp(data, interface)
+            else:
+                # Handle the case where data length is greater than ARP packet length
+                pass
+        elif eth_type == 0x86DD:  # IPv6
+            if len(data) <= self.NDP_PKT_LEN:
+                return self.reply_to_ndp(data, interface)
+            else:
+                # Handle the case where data length is greater than NDP packet length
+                pass
+        else:
+            # Handle other Ethernet types
+            pass
 
     def reply_to_arp(self, data, interface):
         remote_mac, remote_ip, request_ip, op_type, vlan_id = self.extract_arp_info(data)