Properly progress through the list of IPv6 addresses using in6_addr size.

Right now if a jail has multiple IPv6 addresses, it will print them
shifting only 4 bytes at a time. Example:

2001:4dd0:ff41::b23f:a9
2001:4dd0:ff41::b23f:aa

Becomes:

2001:4dd0:ff41::b23f:a9
ff41::b23f:a9:2001:4dd0

By casting to in6_addr, it uses the correct offsets.

MFC after: 1 week