net: flow_dissector: Parse PTP L2 packet header

Add support for parsing PTP L2 packet header. Such packet consists
of an L2 header (with ethertype of ETH_P_1588), PTP header, body
and an optional suffix.

Signed-off-by: Eran Ben Elisha <eranbe@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>