dpaa2-eth: utilize skb->cb[0] for hardware timestamping

This patch is a preparation for next hardware one-step timestamping
support. For DPAA2, the one step timestamping configuration on
hardware registers has to be done when there is no one-step timestamping
packet in flight. So we will have to use workqueue and skb queue
for such packets transmitting, to make sure waiting the last packet has
already been sent on hardware, and starting to transmit the current one.

So the tx timestamping flag in private data may not reflect the actual
request for the one-step timestamping packets of skb queue. This also
affects skb headroom allocation. Let's use skb->cb[0] to mark the
timestamping request for each skb.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>