net: added a BPF to help drivers detect PTP packets.

Certain kinds of hardware time stamping units in both MACs and PHYs have
the limitation that they can only time stamp PTP packets. Drivers for such
hardware are left with the task of correctly matching skbs to time stamps.
This patch adds a BPF that drivers can use to classify PTP packets when
needed.

Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>