ipv6: addrconf: add per netns perturbation in inet6_addr_hash()

Bring IPv6 in par with IPv4 :

- Use net_hash_mix() to spread addresses a bit more.
- Use 256 slots hash table instead of 16

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>