zebra: fix showing nexthop vrf for ipv6 blackhole

[louis-6wind]

For some reasons the Linux kernel associates the ipv6 blackhole of non default table the lo interface.

root@r1# ip -6 route show table 100 root@r1# ip -6 route add unreachable default metric 4278198272 table 100 root@r1# ip -6 route show table 100 unreachable default dev lo metric 4278198272 pref medium

As a consequence, the VRF default that owns the lo interface is shown as the nexthop VRF:

r1# show ipv6 route table 20 Table 20: K>* ::/0 [255/8192] unreachable (ICMP unreachable) (vrf default), 00:18:12

Do not display the nexthop VRF of a blackhole. It does not make sense for a blackhole and it was not displayed in the past.