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: 613d0776 ("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>