make ipv6 address assignment work with tun_linux.go

[JackDoanRivian]

Not backwards compatible with IPv4, but it does work

47: nebula6: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1300 qdisc fq_codel state UNKNOWN group default qlen 500
    link/none
    inet6 fd00::1/9 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::1745:baed:cd2a:ea73/64 scope link stable-privacy proto kernel_ll
       valid_lft forever preferred_lft forever
[jack@jackdesk ~]$ ping -6 fd00::2
PING fd00::2 (fd00::2) 56 data bytes
64 bytes from fd00::2: icmp_seq=1 ttl=64 time=0.721 ms
64 bytes from fd00::2: icmp_seq=2 ttl=64 time=0.622 ms
64 bytes from fd00::2: icmp_seq=3 ttl=64 time=0.612 ms
64 bytes from fd00::2: icmp_seq=4 ttl=64 time=1.46 ms
64 bytes from fd00::2: icmp_seq=5 ttl=64 time=0.648 ms
64 bytes from fd00::2: icmp_seq=6 ttl=64 time=0.636 ms
64 bytes from fd00::2: icmp_seq=7 ttl=64 time=0.553 ms
64 bytes from fd00::2: icmp_seq=8 ttl=64 time=0.896 ms
64 bytes from fd00::2: icmp_seq=9 ttl=64 time=0.514 ms