netfilter: ipt_CLUSTERIP: fix warning unused variable cn

When CONFIG_PROC_FS isn't set the variable cn isn't used.

net/ipv4/netfilter/ipt_CLUSTERIP.c: In function ‘clusterip_net_exit’:
net/ipv4/netfilter/ipt_CLUSTERIP.c:849:24: warning: unused variable ‘cn’ [-Wunused-variable]
  struct clusterip_net *cn = clusterip_pernet(net);
                        ^~

Rework so the variable 'cn' is declared inside "#ifdef CONFIG_PROC_FS".

Fixes: b12f7ba ("netfilter: ipt_CLUSTERIP: remove wrong WARN_ON_ONCE in netns exit routine")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/ipt_CLUSTERIP.c

@@ -846,9 +846,9 @@ static int clusterip_net_init(struct net *net)
 
 static void clusterip_net_exit(struct net *net)
 {
+#ifdef CONFIG_PROC_FS
 	struct clusterip_net *cn = clusterip_pernet(net);
 
-#ifdef CONFIG_PROC_FS
 	mutex_lock(&cn->mutex);
 	proc_remove(cn->procdir);
 	cn->procdir = NULL;