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>

net/ipv6/sysctl_net_ipv6.c

@@ -203,29 +203,16 @@ static int __net_init ipv6_sysctl_net_init(struct net *net)
 	struct ctl_table *ipv6_table;
 	struct ctl_table *ipv6_route_table;
 	struct ctl_table *ipv6_icmp_table;
-	int err;
+	int err, i;
 
 	err = -ENOMEM;
 	ipv6_table = kmemdup(ipv6_table_template, sizeof(ipv6_table_template),
 			     GFP_KERNEL);
 	if (!ipv6_table)
 		goto out;
-	ipv6_table[0].data = &net->ipv6.sysctl.bindv6only;
-	ipv6_table[1].data = &net->ipv6.sysctl.anycast_src_echo_reply;
-	ipv6_table[2].data = &net->ipv6.sysctl.flowlabel_consistency;
-	ipv6_table[3].data = &net->ipv6.sysctl.auto_flowlabels;
-	ipv6_table[4].data = &net->ipv6.sysctl.fwmark_reflect;
-	ipv6_table[5].data = &net->ipv6.sysctl.idgen_retries;
-	ipv6_table[6].data = &net->ipv6.sysctl.idgen_delay;
-	ipv6_table[7].data = &net->ipv6.sysctl.flowlabel_state_ranges;
-	ipv6_table[8].data = &net->ipv6.sysctl.ip_nonlocal_bind;
-	ipv6_table[9].data = &net->ipv6.sysctl.flowlabel_reflect;
-	ipv6_table[10].data = &net->ipv6.sysctl.max_dst_opts_cnt;
-	ipv6_table[11].data = &net->ipv6.sysctl.max_hbh_opts_cnt;
-	ipv6_table[12].data = &net->ipv6.sysctl.max_dst_opts_len;
-	ipv6_table[13].data = &net->ipv6.sysctl.max_hbh_opts_len;
-	ipv6_table[14].data = &net->ipv6.sysctl.multipath_hash_policy,
-	ipv6_table[15].data = &net->ipv6.sysctl.seg6_flowlabel;
+	/* Update the variables to point into the current struct net */
+	for (i = 0; i < ARRAY_SIZE(ipv6_table_template) - 1; i++)
+		ipv6_table[i].data += (void *)net - (void *)&init_net;
 
 	ipv6_route_table = ipv6_route_sysctl_init(net);
 	if (!ipv6_route_table)