net: ena: return 0 in ena_get_rxfh_key_size() when RSS hash key is not configurable

In EC2 instances where the RSS hash key is not configurable, ethtool
shows bogus RSS hash key since ena_get_rxfh_key_size() unconditionally
returns ENA_HASH_KEY_SIZE.

Commit 6a4f7dc ("net: ena: rss: do not allocate key when not
supported") added proper handling for devices that don't support RSS
hash key configuration, but ena_get_rxfh_key_size() has been unchanged.

When the RSS hash key is not configurable, return 0 instead of
ENA_HASH_KEY_SIZE to clarify getting the value is not supported.

Tested on m5 instance families.

Without patch:
 # ethtool -x ens5 | grep -A 1 "RSS hash key"
 RSS hash key:
 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00

With patch:
 # ethtool -x ens5 | grep -A 1 "RSS hash key"
 RSS hash key:
 Operation not supported

Fixes: 6a4f7dc ("net: ena: rss: do not allocate key when not supported")
Signed-off-by: Kohei Enju <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: hyperenju

drivers/net/ethernet/amazon/ena/ena_ethtool.c

@@ -865,7 +865,10 @@ static u32 ena_get_rxfh_indir_size(struct net_device *netdev)
 
 static u32 ena_get_rxfh_key_size(struct net_device *netdev)
 {
-	return ENA_HASH_KEY_SIZE;
+	struct ena_adapter *adapter = netdev_priv(netdev);
+	struct ena_rss *rss = &adapter->ena_dev->rss;
+
+	return rss->hash_key ? ENA_HASH_KEY_SIZE : 0;
 }
 
 static int ena_indirection_table_set(struct ena_adapter *adapter,