ptp: introduce ptp_cancel_worker_sync

In order to effectively use the PTP kernel thread for tasks such as
timestamping packets, allow the user control over stopping it, which is
needed e.g. when the timestamping queues must be drained.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>