net: dsa: Keep a pointer to the skb clone for TX timestamping

For drivers that use deferred_xmit for PTP frames (such as sja1105),
there is no need to perform matching between PTP frames and their egress
timestamps, since the sending process can be serialized.

In that case, it makes sense to have the pointer to the skb clone that
DSA made directly in the skb->cb. It will be used for pushing the egress
timestamp back in the application socket's error queue.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>