ipv6: don't install anycast address for /128 addresses on routers

It does not make sense to create an anycast address for an /128-prefix.
Suppress it.

As 32019e65 ("ipv6: Do not leave router anycast address for /127
prefixes.") shows we also may not leave them, because we could accidentally
remove an anycast address the user has allocated or got added via another
prefix.

Cc: François-Xavier Le Bail <fx.lebail@yahoo.com>
Cc: Thomas Haller <thaller@redhat.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>