nfnetlink_cttimeout: add rcu_barrier() on module removal

Make sure kfree_rcu() released objects before leaving the module removal
exit path.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nfnetlink_cttimeout.c

@@ -612,6 +612,7 @@ static void __exit cttimeout_exit(void)
 	RCU_INIT_POINTER(nf_ct_timeout_find_get_hook, NULL);
 	RCU_INIT_POINTER(nf_ct_timeout_put_hook, NULL);
 #endif /* CONFIG_NF_CONNTRACK_TIMEOUT */
+	rcu_barrier();
 }
 
 module_init(cttimeout_init);