ethtool: Clarify use of size field for ETHTOOL_GRXFHINDIR

In order to find out the device's RX flow hash table size, ethtool
initially uses ETHTOOL_GRXFHINDIR with a buffer size of zero.  This
must be supported, but it is not necessary to support any other user
buffer size less than the device table size.
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/ethtool.h

@@ -543,8 +543,9 @@ struct compat_ethtool_rxnfc {
 /**
  * struct ethtool_rxfh_indir - command to get or set RX flow hash indirection
  * @cmd: Specific command number - %ETHTOOL_GRXFHINDIR or %ETHTOOL_SRXFHINDIR
- * @size: On entry, the array size of the user buffer.  On return from
- *	%ETHTOOL_GRXFHINDIR, the array size of the hardware indirection table.
+ * @size: On entry, the array size of the user buffer, which may be zero
+ *	for %ETHTOOL_GRXFHINDIR.  On return from %ETHTOOL_GRXFHINDIR, the
+ *	array size of the hardware indirection table.
  * @ring_index: RX ring/queue index for each hash value
  */
 struct ethtool_rxfh_indir {