ethtool: fec: sanitize ethtool_fecparam->active_fec

struct ethtool_fecparam::active_fec is a GET-only field,
all in-tree drivers correctly ignore it on SET. Clear
the field on SET to avoid any confusion. Again, we can't
reject non-zero now since ethtool user space does not
zero-init the param correctly.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/ethtool.h

@@ -1378,7 +1378,7 @@ struct ethtool_per_queue_op {
 /**
  * struct ethtool_fecparam - Ethernet forward error correction(fec) parameters
  * @cmd: Command number = %ETHTOOL_GFECPARAM or %ETHTOOL_SFECPARAM
- * @active_fec: FEC mode which is active on the port
+ * @active_fec: FEC mode which is active on the port, GET only.
  * @fec: Bitmask of supported/configured FEC modes
  * @reserved: Reserved for future extensions, ignore on GET, write 0 for SET.
  */

net/ethtool/ioctl.c

@@ -2586,6 +2586,7 @@ static int ethtool_set_fecparam(struct net_device *dev, void __user *useraddr)
 	if (copy_from_user(&fecparam, useraddr, sizeof(fecparam)))
 		return -EFAULT;
 
+	fecparam.active_fec = 0;
 	fecparam.reserved = 0;
 
 	return dev->ethtool_ops->set_fecparam(dev, &fecparam);