[swss][arp_update] Send ipv6 pings over vlan sub interfaces (#8363)

#### Why I did it
* `arp_update` fails to ping those neighbors over vlan sub interfaces.

#### How I did it
* modify `arp_update_vars.j2` to get vlan sub interfaces with ipv6 addresses assigned.
* modify `arp_update` to send ipv6 pings over those retrieved vlan sub interfaces.

Signed-off-by: Longxiang Lyu <lolv@microsoft.com>

files/build_templates/arp_update_vars.j2

@@ -1,5 +1,6 @@
 {
     "interface": "{% for (name, prefix) in INTERFACE|pfx_filter %}{% if prefix|ipv6 %}{{ name }} {% endif %}{% endfor %}",
     "pc_interface" : "{% for (name, prefix) in PORTCHANNEL_INTERFACE|pfx_filter %}{% if prefix|ipv6 %}{{ name }} {% endif %}{% endfor %}",
+    "vlan_sub_interface": "{% for (name, prefix) in VLAN_SUB_INTERFACE|pfx_filter %}{% if prefix|ipv6 %}{{ name }} {% endif %}{% endfor %}",
     "vlan" : "{% if VLAN %}{{ VLAN.keys() | join(' ') }}{% endif %}"
 }

files/scripts/arp_update

@@ -14,8 +14,9 @@ while /bin/true; do
   ARP_UPDATE_VARS=$(sonic-cfggen -d -t ${ARP_UPDATE_VARS_FILE})
   INTERFACE=$(echo $ARP_UPDATE_VARS | jq -r '.interface')
   PC_INTERFACE=$(echo $ARP_UPDATE_VARS | jq -r '.pc_interface')
+  VLAN_SUB_INTERFACE=$(echo $ARP_UPDATE_VARS | jq -r '.vlan_sub_interface')
 
-  ALL_INTERFACE="$INTERFACE $PC_INTERFACE"
+  ALL_INTERFACE="$INTERFACE $PC_INTERFACE $VLAN_SUB_INTERFACE"
   for intf in $ALL_INTERFACE; do
       ping6cmd="ping6 -I $intf -n -q -i 0 -c 1 -W 0 ff02::1 >/dev/null"
       intf_up=$(ip link show $intf | grep "state UP")