net: dsa: don't call ptp_classify_raw() if switch doesn't provide RX timestamping

ptp_classify_raw() is not exactly cheap, since it invokes a BPF program
for every skb in the receive path. For switches which do not provide
ds->ops->port_rxtstamp(), running ptp_classify_raw() provides precisely
nothing, so check for the presence of the function pointer first, since
that is much cheaper.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Link: https://lore.kernel.org/r/20221209175840.390707-1-vladimir.oltean@nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/tag.c

@@ -33,6 +33,9 @@ static bool dsa_skb_defer_rx_timestamp(struct dsa_slave_priv *p,
 	struct dsa_switch *ds = p->dp->ds;
 	unsigned int type;
 
+	if (!ds->ops->port_rxtstamp)
+		return false;
+
 	if (skb_headroom(skb) < ETH_HLEN)
 		return false;
 
@@ -45,10 +48,7 @@ static bool dsa_skb_defer_rx_timestamp(struct dsa_slave_priv *p,
 	if (type == PTP_CLASS_NONE)
 		return false;
 
-	if (likely(ds->ops->port_rxtstamp))
-		return ds->ops->port_rxtstamp(ds, p->dp->index, skb, type);
-
-	return false;
+	return ds->ops->port_rxtstamp(ds, p->dp->index, skb, type);
 }
 
 static int dsa_switch_rcv(struct sk_buff *skb, struct net_device *dev,