Eliminate incorrect IPv6 mask guessing:
RFC 2374 concept of 'IPv6 Aggregatable Global Unicast Address Format' was
deprecated by RFC 3587 12 years ago.

Before:

15:06 [1] edge# netstat -rn6 | grep 2a02:6b8::
2a02:6b8::/32 2a02:978:2::1 UGS em0
15:06 [1] edge# route -6n get 2a02:6b8::
route: writing to routing socket: No such process

After:
15:07 [1] edge# /usr/obj/usr/src/sbin/route/route -n6 get 2a02:6b8::

route to: 2a02:6b8::

destination: 2a02:6b8::

     mask: ffff:ffff::
  gateway: 2a02:978:2::1
      fib: 0
interface: em0
    flags: <UP,GATEWAY,DONE,STATIC>

recvpipe sendpipe ssthresh rtt,msec mtu weight expire

0         0         0         0      1500         1         0

MFC after: 2 weeks