ethtool: Define and apply a default policy for RX flow hash indirection

All drivers that support modification of the RX flow hash indirection
table initialise it in the same way: RX rings are assigned to table
entries in rotation.  Make that default policy explicit by having them
call a ethtool_rxfh_indir_default() function.

In the ethtool core, add support for a zero size value for
ETHTOOL_SRXFHINDIR, which resets the table to this default.

Partly-suggested-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: Shreyas N Bhatewara <sbhatewara@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vmxnet3/vmxnet3_drv.c

@@ -2167,7 +2167,8 @@ vmxnet3_setup_driver_shared(struct vmxnet3_adapter *adapter)
 		rssConf->indTableSize = VMXNET3_RSS_IND_TABLE_SIZE;
 		get_random_bytes(&rssConf->hashKey[0], rssConf->hashKeySize);
 		for (i = 0; i < rssConf->indTableSize; i++)
-			rssConf->indTable[i] = i % adapter->num_rx_queues;
+			rssConf->indTable[i] = ethtool_rxfh_indir_default(
+				i, adapter->num_rx_queues);
 
 		devRead->rssConfDesc.confVer = 1;
 		devRead->rssConfDesc.confLen = sizeof(*rssConf);