bonding: fix netdev event NULL pointer dereference

In commit 471cb5a3 ("bonding: remove
usage of dev->master") a bug was introduced which causes a NULL pointer
dereference. If a bond device is in mode 6 (ALB) and a slave is added
it will dereference a NULL pointer in bond_slave_netdev_event().
This is because in bond_enslave we have bond_alb_init_slave() which
changes the MAC address of the slave and causes a NETDEV_CHANGEADDR.
Then we have in bond_slave_netdev_event():
        struct slave *slave = bond_slave_get_rtnl(slave_dev);
        struct bonding *bond = slave->bond;
bond_slave_get_rtnl() dereferences slave_dev->rx_handler_data which at
that time is NULL since netdev_rx_handler_register() is called later.

This is fixed by checking if slave is NULL before dereferencing it.

v2: Comment style changed.
Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -3168,11 +3168,20 @@ static int bond_slave_netdev_event(unsigned long event,
 				   struct net_device *slave_dev)
 {
 	struct slave *slave = bond_slave_get_rtnl(slave_dev);
-	struct bonding *bond = slave->bond;
-	struct net_device *bond_dev = slave->bond->dev;
+	struct bonding *bond;
+	struct net_device *bond_dev;
 	u32 old_speed;
 	u8 old_duplex;
 
+	/* A netdev event can be generated while enslaving a device
+	 * before netdev_rx_handler_register is called in which case
+	 * slave will be NULL
+	 */
+	if (!slave)
+		return NOTIFY_DONE;
+	bond_dev = slave->bond->dev;
+	bond = slave->bond;
+
 	switch (event) {
 	case NETDEV_UNREGISTER:
 		if (bond->setup_by_slave)