netfilter: ipt_CLUSTERIP: fix clusterip_net_exit build regression

The added check produces a build error when CONFIG_PROC_FS is
disabled:

net/ipv4/netfilter/ipt_CLUSTERIP.c: In function 'clusterip_net_exit':
net/ipv4/netfilter/ipt_CLUSTERIP.c:822:28: error: 'cn' undeclared (first use in this function)

This moves the variable declaration out of the #ifdef to make it
available to the WARN_ON_ONCE().

Fixes: 613d0776d3fe ("netfilter: exit_net cleanup check added")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/ipt_CLUSTERIP.c

@@ -813,8 +813,8 @@ static int clusterip_net_init(struct net *net)
 
 static void clusterip_net_exit(struct net *net)
 {
-#ifdef CONFIG_PROC_FS
 	struct clusterip_net *cn = net_generic(net, clusterip_net_id);
+#ifdef CONFIG_PROC_FS
 	proc_remove(cn->procdir);
 	cn->procdir = NULL;
 #endif