net: mscc: ocelot: add locking for the port TX timestamp ID

The ocelot_port->ts_id is used to:
(a) populate skb->cb[0] for matching the TX timestamp in the PTP IRQ
    with an skb.
(b) populate the REW_OP from the injection header of the ongoing skb.
Only then is ocelot_port->ts_id incremented.

This is a problem because, at least theoretically, another timestampable
skb might use the same ocelot_port->ts_id before that is incremented.
Normally all transmit calls are serialized by the netdev transmit
spinlock, but in this case, ocelot_port_add_txtstamp_skb() is also
called by DSA, which has started declaring the NETIF_F_LLTX feature
since commit 2b86cb829976 ("net: dsa: declare lockless TX feature for
slave ports").  So the logic of using and incrementing the timestamp id
should be atomic per port.

The solution is to use the global ocelot_port->ts_id only while
protected by the associated ocelot_port->ts_id_lock. That's where we
populate skb->cb[0]. Note that for ocelot, ocelot_port_add_txtstamp_skb
is called for the actual skb, but for felix, it is called for the skb's
clone. That is something which will also be changed in the future.

Signed-off-by: Vladimir Oltean <[email protected]>
Reviewed-by: Horatiu Vultur <[email protected]>
Reviewed-by: Florian Fainelli <[email protected]>
Tested-by: Alexandre Belloni <[email protected]>
Reviewed-by: Alexandre Belloni <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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