net: ptp: add helper for one-step P2P clocks

For P2P delay measurement, the ingress time stamp of the PDelay_Req is
required for the correction field of the PDelay_Resp. The application
echoes back the correction field of the PDelay_Req when sending the
PDelay_Resp.

Some hardware (like the ZHAW InES PTP time stamping IP core) subtracts
the ingress timestamp autonomously from the correction field, so that
the hardware only needs to add the egress timestamp on tx. Other
hardware (like the Microchip KSZ9563) reports the ingress time stamp via
an interrupt and requires that the software provides this time stamp via
tail-tag on tx.

In order to avoid introducing a further application interface for this,
the driver can simply emulate the behavior of the InES device and
subtract the ingress time stamp in software from the correction field.

On egress, the correction field can either be kept as it is (and the
time stamp field in the tail-tag is set to zero) or move the value from
the correction field back to the tail-tag.

Changing the correction field requires updating the UDP checksum (if UDP
is used as transport).

Signed-off-by: Christian Eggers <ceggers@arri.de>
Co-developed-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>