Fix compilation for 5.9: workaround ref_module unexport

compat.h:173:21: error: implicit declaration of function ‘ref_module’ [-Werror=implicit-function-declaration]
 # define use_module ref_module
                     ^
ipt_NETFLOW.c:5488:3: note: in expansion of macro ‘use_module’
   use_module(THIS_MODULE, netlink_m);
   ^~~~~~~~~~

Fixes #153.
Reported-by: https://github.com/rcmcronny

Author: aabc

ipt_NETFLOW.c

@@ -5459,13 +5459,12 @@ static struct pernet_operations natevents_net_ops = {
 #endif /* since 2.6.31 */
 
 static DEFINE_MUTEX(events_lock);
+static struct module *netlink_m;
 /* Both functions may be called multiple times. */
 static void register_ct_events(void)
 {
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,31)
 #define NETLINK_M "nf_conntrack_netlink"
-	struct module *netlink_m;
-	static int referenced = 0;
 #endif
 
 	printk(KERN_INFO "ipt_NETFLOW: enable natevents.\n");
@@ -5478,14 +5477,19 @@ static void register_ct_events(void)
 #if LINUX_VERSION_CODE < KERNEL_VERSION(3,2,0)
 	    !rcu_dereference(nf_conntrack_event_cb) ||
 #endif
-	    !(netlink_m = find_module(NETLINK_M))) {
+	    !find_module(NETLINK_M)) {
 		printk("Loading " NETLINK_M "\n");
 		request_module(NETLINK_M);
+
 	}
 	/* Reference netlink module to prevent it's unsafe unload before us. */
-	if (!referenced && (netlink_m = find_module(NETLINK_M))) {
-		referenced++;
+	if (!netlink_m && (netlink_m = find_module(NETLINK_M))) {
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5,9,0)
 		use_module(THIS_MODULE, netlink_m);
+#else
+		if (!try_module_get(netlink_m))
+			netlink_m = NULL;
+#endif
 	}
 
 	/* Register ct events callback. */
@@ -5513,6 +5517,10 @@ static void unregister_ct_events(void)
 #else /* < v3.2 */
 	unset_notifier_cb();
 #endif /* v3.2 */
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5,9,0)
+	module_put(netlink_m);
+	netlink_m = NULL;
+#endif
 	rcu_assign_pointer(saved_event_cb, NULL);
 #else /* < v2.6.31 */
 	nf_conntrack_unregister_notifier(&ctnl_notifier);