net: Convert ip6 tables pernet_operations

The pernet_operations:

    ip6table_filter_net_ops
    ip6table_mangle_net_ops
    ip6table_nat_net_ops
    ip6table_raw_net_ops
    ip6table_security_net_ops

have exit methods, which call ip6t_unregister_table().
ip6table_filter_net_ops has init method registering
filter table.

Since there must not be in-flight ipv6 packets at the time
of pernet_operations execution and since pernet_operations
don't send ipv6 packets each other, these pernet_operations
are safe to be async.

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