net: lapbether: use netdev_lockdep_set_classes() helper

drivers/net/wan/lapbether.c uses stacked devices.
Like similar drivers, it must use netdev_lockdep_set_classes()
to avoid LOCKDEP splats.

This is similar to commit 9bfc9d6 ("hamradio:
use netdev_lockdep_set_classes() helper")

Fixes: 7e4d784 ("net: hold netdev instance lock during rtnetlink operations")
Reported-by: syzbot+377b71db585c9c705f8e@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/67cd611c.050a0220.14db68.0073.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250327144439.2463509-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/wan/lapbether.c

@@ -39,6 +39,7 @@
 #include <linux/lapb.h>
 #include <linux/init.h>
 
+#include <net/netdev_lock.h>
 #include <net/x25device.h>
 
 static const u8 bcast_addr[6] = { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF };
@@ -366,6 +367,7 @@ static const struct net_device_ops lapbeth_netdev_ops = {
 
 static void lapbeth_setup(struct net_device *dev)
 {
+	netdev_lockdep_set_classes(dev);
 	dev->netdev_ops	     = &lapbeth_netdev_ops;
 	dev->needs_free_netdev = true;
 	dev->type            = ARPHRD_X25;