net: ethtool: Initialize buffer when querying device channel settings

Ethtool channels respond struct was uninitialized when querying device
channel boundaries settings. As a result, unreported fields by the driver
hold garbage.  This may cause sending unsupported params to driver.

Fixes: 8bf368620486 ('ethtool: ensure channel counts are within bounds ...')
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
CC: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/ethtool.c

@@ -1712,7 +1712,7 @@ static noinline_for_stack int ethtool_get_channels(struct net_device *dev,
 static noinline_for_stack int ethtool_set_channels(struct net_device *dev,
 						   void __user *useraddr)
 {
-	struct ethtool_channels channels, max;
+	struct ethtool_channels channels, max = { .cmd = ETHTOOL_GCHANNELS };
 	u32 max_rx_in_use = 0;
 
 	if (!dev->ethtool_ops->set_channels || !dev->ethtool_ops->get_channels)