net: dsa: Keep private info in the skb->cb

Map a DSA structure over the 48-byte control block that will hold
skb info on transmit and receive. This is only for use within the DSA
processing layer (e.g. communicating between DSA core and tagger) and
not for passing info around with other layers such as the master net
device.

Also add a DSA_SKB_CB_PRIV() macro which retrieves a pointer to the
space up to 48 bytes that the DSA structure does not use. This space can
be used for drivers to add their own private info.

One use is for the PTP timestamping code path. When cloning a skb,
annotate the original with a pointer to the clone, which the driver can
then find easily and place the timestamp to. This avoids the need of a
separate queue to hold clones and a way to match an original to a cloned
skb.

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>