flow: Enable matching on new field 'pkt_mark'.

flow: Enable matching on new field 'pkt_mark'.

The Linux kernel datapath enables matching and setting the skb mark
but this functionality is currently used only internally by
ovs-vswitchd. This exposes it through NXM to enable external
controllers to interact with other kernel subsystems. Although this
is simply exporting the skb mark, the intention is that this is a
platform independent mechanism to access some system metadata and
therefore may have different implementations on various systems.

Bug #17855

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>