net: dsa: avoid potential use-after-free error

If dsa_switch_ops::port_txtstamp() returns false, clone will be freed
immediately. Shouldn't store a pointer to freed memory.
Signed-off-by: Christian Eggers <ceggers@arri.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20201119110906.25558-1-ceggers@arri.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/slave.c

@@ -522,10 +522,10 @@ static void dsa_skb_tx_timestamp(struct dsa_slave_priv *p,
 	if (!clone)
 		return;
 
-	DSA_SKB_CB(skb)->clone = clone;
-
-	if (ds->ops->port_txtstamp(ds, p->dp->index, clone, type))
+	if (ds->ops->port_txtstamp(ds, p->dp->index, clone, type)) {
+		DSA_SKB_CB(skb)->clone = clone;
 		return;
+	}
 
 	kfree_skb(clone);
 }