software/ors-amarisoft: Do not give RU more than /64

Dnsmasq insists on dhcp-range's prefixlen to be at most 64, which
triggers the following error if original slaptap is wider than that:

    dnsmasq: prefix length must be at least 64 at line 5 of /srv/slapgrid/slappart6/etc/dnsmasq.cfg

-> Fix it by capping provided range to /64

/reported-by @lu.xu
/cc @jhuge, @tomo, @xavier_thompson, @Daetalus

software/ors-amarisoft/ru/dnsmasq.jinja2.cfg

@@ -7,7 +7,8 @@ port=5354
 {%-   set plen = netaddr.IPNetwork(vtap.network).prefixlen  %}
 
 # {{ cell_ref }} @ {{ ru_tap }}
-dhcp-range=tag:{{ ru_tap }},{{ vtap.gateway }},{{ vtap.gateway }},static,{{ plen }},5m
+{#- TODO consider using /128 as we give only 1 address to RU #}
+dhcp-range=tag:{{ ru_tap }},{{ vtap.gateway }},{{ vtap.gateway }},static,{{ max(plen,64) }},5m
 dhcp-host={{ cell.ru_mac_addr }},tag:{{ ru_tap }},[{{ vtap.gateway }}]
 # option 17 used for RU callhome
 # dhcp-option=option6:17,[{{ vtap.addr }}]