drbd: remove /proc/drbd before unregistering from netlink

There still exists a (theoretical) race on module unload, where
/proc/drbd may still exist, but the netlink callback has been
unregistered already, allowing drbdsetup to shout without listeners,
and get no reply.

Reorder remove_proc_entry and unregister of netlink callback.
drbdsetup first checks for existence of the proc entry,
and if that is missing, won't even try to contact the module.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_main.c

@@ -3174,11 +3174,20 @@ static void drbd_cleanup(void)
 
 	unregister_reboot_notifier(&drbd_notifier);
 
+	/* first remove proc,
+	 * drbdsetup uses it's presence to detect
+	 * whether DRBD is loaded.
+	 * If we would get stuck in proc removal,
+	 * but have netlink already deregistered,
+	 * some drbdsetup commands may wait forever
+	 * for an answer.
+	 */
+	if (drbd_proc)
+		remove_proc_entry("drbd", NULL);
+
 	drbd_nl_cleanup();
 
 	if (minor_table) {
-		if (drbd_proc)
-			remove_proc_entry("drbd", NULL);
 		i = minor_count;
 		while (i--)
 			drbd_delete_device(i);