scripts: dhcpv6: don't report custom ip6class if default

odhcp6c returns delegated prefixes with a "class=<IAID>" tag if a custom
IAID was specified in the -P argument. This tag must then be used for any
prefix filter ("network.<interface>.ip6class") instead of the uplink i/f
name.

OpenWrt by default uses the first eight digits of the uplink i/f name's MD5
hash as IAID. Don't report a custom "class" tag for this default IAID.

Fixes: f08cd5ce5d
Signed-off-by: Shine <4c.fce2@proton.me>
Tested-by: Goetz Goerisch <ggoerisch@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23989
Signed-off-by: Robert Marko <robimarko@gmail.com>

package/network/ipv6/odhcp6c/files/dhcpv6.script

@@ -1,10 +1,12 @@
 #!/bin/sh
 [ -z "$2" ] && echo "Error: should be run by odhcpc6c" && exit 1
 . /lib/functions.sh
+. /lib/functions/network.sh
 . /lib/netifd/netifd-proto.sh
 
 setup_interface () {
 	local device="$1"
+	local iaid
 	local prefsig=""
 	local addrsig=""
 
@@ -42,7 +44,9 @@ setup_interface () {
 		proto_add_dns_search "$domain"
 	done
 
+	network_generate_iface_iaid iaid "$device"
 	for prefix in $PREFIXES; do
+		prefix="${prefix/,class=$iaid/}"
 		proto_add_ipv6_prefix "$prefix"
 		prefsig="$prefsig ${prefix%%,*}"
 		local entry="${prefix#*/}"