net: Convert ipv6_sysctl_net_ops

These pernet_operations create and destroy sysctl tables.
They are not touched by another net pernet_operations.
So, it's possible to execute them in parallel with others.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>