bonding: allow nesting of bonding device

The commit 3c9ef51 ("bonding: avoid adding slave device with
IFF_MASTER flag") fix a crash when add slave device with IFF_MASTER,
but it rejects the scenario of nested bonding device.

As Eric Dumazet described: since there indeed is a usage scenario about
nesting bonding, we should not break it.

So we add a new judgment condition to allow nesting of bonding device.

Fixes: 3c9ef51 ("bonding: avoid adding slave device with IFF_MASTER flag")
Suggested-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: Di Zhu <zhudi21@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -1601,7 +1601,9 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 	int link_reporting;
 	int res = 0, i;
 
-	if (slave_dev->flags & IFF_MASTER) {
+	if (slave_dev->flags & IFF_MASTER &&
+	    !netif_is_bond_master(slave_dev)) {
+		NL_SET_ERR_MSG(extack, "Device with IFF_MASTER cannot be enslaved");
 		netdev_err(bond_dev,
 			   "Error: Device with IFF_MASTER cannot be enslaved\n");
 		return -EPERM;