[202305] Fix v6relay dual tor if selection issue (#17186)

Why I did it
dhcp_relay dual tor tests are failing in 202305

How I did it
Backport #15864 to 202305

How to verify it
Ran sonic-mgmt dhcp_relay tests.

dockers/docker-dhcp-relay/dhcpv6-relay.agents.j2

@@ -13,7 +13,7 @@
 [program:dhcp6relay]
 command=/usr/sbin/dhcp6relay
 {#- Dual ToR Option #}
-{% if 'subtype' in DEVICE_METADATA['localhost'] and DEVICE_METADATA['localhost']['subtype'] == 'DualToR' %} -d{% endif %}
+{% if 'subtype' in DEVICE_METADATA['localhost'] and DEVICE_METADATA['localhost']['subtype'] == 'DualToR' %} -u Loopback0 {% endif %}
 
 priority=3
 autostart=false