ipv6: assign rt6_info to inet6_ifaddr in init_loopback

Commit 25fb6ca4
"net IPv6 : Fix broken IPv6 routing table after loopback down-up"
forgot to assign rt6_info to the inet6_ifaddr.
When disable the net device, the rt6_info which allocated
in init_loopback will not be destroied in __ipv6_ifa_notify.

This will trigger the waring message below
[23527.916091] unregister_netdevice: waiting for tap0 to become free. Usage count = 1

Reported-by: Arkadiusz Miskiewicz <a.miskiewicz@gmail.com>
Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>