net: Add net_dev_start_xmit trace event, exposing more skb fields

The existing net/net_dev_xmit trace event provides little information
about the skb that has been passed to the driver, and it is not
simple to add more since the skb may already have been freed at
the point the event is emitted.

Add a separate trace event before the skb is passed to the driver,
including most fields that are likely to be interesting for debugging
driver datapath behaviour.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>