bonding: fix a potential double-unregister

When we tear down a network namespace, we unregister all
the netdevices within it. So we may queue a slave device
and a bonding device together in the same unregister queue.

If the only slave device is non-ethernet, it would
automatically unregister the bonding device as well. Thus,
we may end up unregistering the bonding device twice.

Workaround this special case by checking reg_state.

Fixes: 9b5e383c ("net: Introduce unregister_netdevice_many()")
Reported-by: syzbot+af23e7f3e0a7e10c8b67@syzkaller.appspotmail.com
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Andy Gospodarek <andy@greyhouse.net>
Cc: Jay Vosburgh <j.vosburgh@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -2207,7 +2207,8 @@ static int bond_release_and_destroy(struct net_device *bond_dev,
 	int ret;
 
 	ret = __bond_release_one(bond_dev, slave_dev, false, true);
-	if (ret == 0 && !bond_has_slaves(bond)) {
+	if (ret == 0 && !bond_has_slaves(bond) &&
+	    bond_dev->reg_state != NETREG_UNREGISTERING) {
 		bond_dev->priv_flags |= IFF_DISABLE_NETPOLL;
 		netdev_info(bond_dev, "Destroying bond\n");
 		bond_remove_proc_entry(bond);