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().