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

/cc @tomo, @xavier_thompson, @Daetalus
/reported-by @lu.xu
/reviewed-by @jhuge
/reviewed-on nexedi/slapos!1509