failover: Add missing check to validate 'slave_dev' in net_failover_slave_unregister

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/net_failover.c: In function 'net_failover_slave_unregister':
drivers/net/net_failover.c:598:35: warning:
 variable 'primary_dev' set but not used [-Wunused-but-set-variable]

There should check the validity of 'slave_dev'.

Fixes: cfc80d9a ("net: Introduce net_failover driver")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/net_failover.c

@@ -602,6 +602,9 @@ static int net_failover_slave_unregister(struct net_device *slave_dev,
 	primary_dev = rtnl_dereference(nfo_info->primary_dev);
 	standby_dev = rtnl_dereference(nfo_info->standby_dev);
 
+	if (WARN_ON_ONCE(slave_dev != primary_dev && slave_dev != standby_dev))
+		return -ENODEV;
+
 	vlan_vids_del_by_dev(slave_dev, failover_dev);
 	dev_uc_unsync(slave_dev, failover_dev);
 	dev_mc_unsync(slave_dev, failover_dev);