net: dsa: mv88e6xxx: Fix receive time stamp race condition.

The DSA stack passes received PTP frames to this driver via
mv88e6xxx_port_rxtstamp() for deferred delivery.  The driver then
queues the frame and kicks the worker thread.  The work callback reads
out the latched receive time stamp and then works through the queue,
delivering any non-matching frames without a time stamp.

If a new frame arrives after the worker thread has read out the time
stamp register but enters the queue before the worker finishes
processing the queue, that frame will be delivered without a time
stamp.

This patch fixes the race by moving the queue onto a list on the stack
before reading out the latched time stamp value.

Fixes: c6fe0ad2 ("net: dsa: mv88e6xxx: add rx/tx timestamping support")
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>