The following patch applies on top of Maksim's one. It:
- adds a missing include (hunk #1);
- turns a cast fiesta into the equivalent list_entry (hunk #2);
- unregisters net_device once it isn't needed for bridging (hunk #3);
- compiles and stands a kill-br2684ctl-before-ifconfig-down test: nas0 
  disappeared as it should.
Without the patch an oops happens, see:
<URL:http://www.cogenit.fr/linux/atm/oopses/text-6>
Comments welcome.
--- net/atm/br2684.c.orig	Sat Mar 23 21:21:41 2002
+++ net/atm/br2684.c	Sun Mar 24 21:42:30 2002
@@ -15,6 +15,7 @@ Author: Marcell GAL, 2000, XDSL Ltd, Hun
 #include <linux/etherdevice.h>
 #include <net/arp.h>
 #include <linux/rtnetlink.h>
+#include <linux/ip.h>
 #include <linux/atmbr2684.h>
 
 #include "ipcommon.h"
@@ -97,8 +98,7 @@ static LIST_HEAD(br2684_devs);
 
 static inline struct br2684_dev *BRPRIV(const struct net_device *net_dev)
 {
-	return (struct br2684_dev *) ((char *) (net_dev) -
-	    (unsigned long) (&((struct br2684_dev *) 0)->net_dev));
+	return list_entry(net_dev, struct br2684_dev, net_dev);
 }
 
 static inline struct br2684_dev *list_entry_brdev(const struct list_head *le)
@@ -410,6 +410,13 @@ static void br2684_push(struct atm_vcc *
 
 	if (skb == NULL) {	/* skb==NULL means VCC is being destroyed */
 		br2684_close_vcc(brvcc);
+		if (list_empty(&brdev->brvccs)) {
+			read_lock(&devs_lock);
+			list_del(&brdev->br2684_devs);
+			read_unlock(&devs_lock);
+			unregister_netdev(&brdev->net_dev);
+			kfree(brdev);
+		}
 		return;
 	}
 
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