net_failover: fixed rollback in net_failover_open()

found by smatch:
drivers/net/net_failover.c:65 net_failover_open() error:
 we previously assumed 'primary_dev' could be null (see line 43)

Fixes: cfc80d9a ("net: Introduce net_failover driver")
Signed-off-by: NVasily Averin <vvs@virtuozzo.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/net_failover.c

@@ -61,7 +61,8 @@ static int net_failover_open(struct net_device *dev)
 	return 0;
 
 err_standby_open:
-	dev_close(primary_dev);
+	if (primary_dev)
+		dev_close(primary_dev);
 err_primary_open:
 	netif_tx_disable(dev);
 	return err;