Fix the issue introduced when handle the 2 PRs sonic-net#16948 and sonic-net#16970 (sonic-net#17543)

Author: nhe-NV

tests/common/fixtures/ptfhost_utils.py

@@ -210,15 +210,16 @@ def setup_vlan_arp_responder(ptfhost, rand_selected_dut, tbinfo):
     if 'dualtor' in tbinfo['topo']['name']:
         server_ip = mux_cable_server_ip(rand_selected_dut)
 
+    ip_offset = 0
     for port in vlan_members:
         ptf_index = dut_to_ptf_port_map[port]
+        ip_offset = ptf_index + 1  # Add one since PTF indices start at 0
         if 'dualtor' in tbinfo['topo']['name']:
             arp_responder_cfg['eth{}'.format(ptf_index)] = [
                 server_ip[port]['server_ipv4'].split('/')[0],
                 server_ip[port]['server_ipv6'].split('/')[0]
             ]
             continue
-        ip_offset = ptf_index + 1  # Add one since PTF indices start at 0
         arp_responder_cfg['eth{}'.format(ptf_index)] = [
             str(ipv4_base.ip + ip_offset), str(ipv6_base.ip + ip_offset)
         ]