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: cfc80d9a1163 ("net: Introduce net_failover driver")
Signed-off-by: Vasily Averin <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/net_failover.c

···
 	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;