mlxsw: pci: PTP: Hook into packet transmit path

On Spectrum-1, timestamps are delivered separately from the packets, and
need to paired up. Therefore, at some point after mlxsw_sp_port_xmit()
is invoked, it is necessary to involve the chip-specific driver code to
allow it to do the necessary bookkeeping and matching.

On Spectrum-2, timestamps are delivered in CQE. For that reason,
position the point of driver involvement into mlxsw_pci_cqe_sdq_handle()
to make it hopefully easier to extend for Spectrum-2 in the future.

To tell the driver what port the packet was sent on, keep tx_info
in SKB control buffer.

Introduce a new driver core interface mlxsw_core_ptp_transmitted(), a
driver callback ptp_transmitted, and a PTP op transmitted. The callee is
responsible for taking care of releasing the SKB passed to the new
interfaces, and correspondingly have the new stub callbacks just call
dev_kfree_skb_any().

Follow-up patches will introduce the actual content into
mlxsw_sp1_ptp_transmitted() in particular.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>