ipv6: Use math to point per net sysctls into the appropriate struct net

The data pointers of ipv6 sysctl are set one by one which is hard to
maintain, especially with kconfig. This patch simplifies it by using
math to point the per net sysctls into the appropriate struct net,
just like what we did for ipv4.

Signed-off-by: Cambda Zhu <cambda@linux.alibaba.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>