pkt_cls: add new tc cls helper to check offload flag and chain index

Very few (mlxsw) upstream drivers seem to allow offload of chains
other than 0.  Save driver developers typing and add a helper for
checking both if ethtool's TC offload flag is on and if chain is 0.
This helper will set the extack appropriately in both error cases.
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/pkt_cls.h

@@ -656,6 +656,20 @@ static inline bool tc_can_offload_extack(const struct net_device *dev,
 	return can;
 }
 
+static inline bool
+tc_cls_can_offload_and_chain0(const struct net_device *dev,
+			      struct tc_cls_common_offload *common)
+{
+	if (!tc_can_offload_extack(dev, common->extack))
+		return false;
+	if (common->chain_index) {
+		NL_SET_ERR_MSG(common->extack,
+			       "Driver supports only offload of chain 0");
+		return false;
+	}
+	return true;
+}
+
 static inline bool tc_skip_hw(u32 flags)
 {
 	return (flags & TCA_CLS_FLAGS_SKIP_HW) ? true : false;