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 <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Reviewed-by: Kurt Kanzenbach <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/dsa/tag.c

···
 	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;
 
···
 	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,