PPP IPV6CP protocol exchanges only IPv6 Interface Identifiers which are
used for generating IPv6 link-local addresses. As on PPP link cannot be
more hosts and IPv6 link-local addresses are designed for addressing a
single link, set route prefix for local and remote IPv6 link-local
addresses to /128 which means single IPv6 address.
This ensures that Linux kernel would not try to route other IPv6 link-local
addresses over PPP link.
Routable IPv6 prefix on PPP link is later configured either by ICMPv6
Router Advertisement packets or manually/statically. Routable local IPv6
address is then assigned by SLAAC, DHCPv6 or manual/static configuration.
So this change has no effect for routable IPv6 address or routable IPv6
prefix.
Note that pppd for IPv4 already sets netmask to 255.255.255.255, one single
IPv4 address.