ethtool: fec: add note about reuse of reserved

struct ethtool_fecparam::reserved can't be used in SET, because
ethtool user space doesn't zero-initialize the structure.
Make this clear.
Suggested-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/uapi/linux/ethtool.h

@@ -1382,6 +1382,10 @@ struct ethtool_per_queue_op {
  * @fec: Bitmask of configured FEC modes.
  * @reserved: Reserved for future extensions, ignore on GET, write 0 for SET.
  *
+ * Note that @reserved was never validated on input and ethtool user space
+ * left it uninitialized when calling SET. Hence going forward it can only be
+ * used to return a value to userspace with GET.
+ *
  * FEC modes supported by the device can be read via %ETHTOOL_GLINKSETTINGS.
  * FEC settings are configured by link autonegotiation whenever it's enabled.
  * With autoneg on %ETHTOOL_GFECPARAM can be used to read the current mode.