use variable for tun0 interface

conf/scripts/route-down.d/40-vpnclient-unset-ipv6

@@ -2,10 +2,12 @@
 
 is_ip6addr_set() {
   local ip6_addr=${1}
-  ip address show dev tun0 2> /dev/null | grep -q "${ip6_addr}/"
+  ip address show dev "${gateway_interface}" 2> /dev/null | grep -q "${ip6_addr}/"
 }
 
 ip6_addr=$(yunohost app setting "vpnclient" "ip6_addr")
+gateway_interface=${dev}
+
 if [[ -n "${ip6_addr}" ]] && [[ "${ip6_addr}" != none ]] && is_ip6addr_set "${ip6_addr}"; then
-  ip address delete "${ip6_addr}/64" dev tun0
+  ip address delete "${ip6_addr}/64" dev "${gateway_interface}"
 fi

conf/scripts/route-up.d/40-vpnclient-set-ipv6

@@ -2,13 +2,15 @@
 
 is_ip6addr_set() {
   local ip6_addr=${1}
-  ip address show dev tun0 2> /dev/null | grep -q "${ip6_addr}/"
+  ip address show dev "${gateway_interface}" 2> /dev/null | grep -q "${ip6_addr}/"
 }
 
 ip6_addr=$(yunohost app setting "vpnclient" "ip6_addr")
+gateway_interface=${dev}
+
 if [[ -n "${ip6_addr}" ]] && [[ "${ip6_addr}" != none ]]; then
   if ! is_ip6addr_set "${ip6_addr}"; then
-    ip address add "${ip6_addr}/64" dev tun0
+    ip address add "${ip6_addr}/64" dev "${gateway_interface}"
   fi
 
   echo "[INFO] IPv6 delegated prefix found"