ovs-dpctl: Fix mega flow output

ovs-dpctl: Fix mega flow output

ovs-dpctl sometimes displays wildcarded fields as exact match. This
patch fixes those cases.

This patch implements the following logic. When OVS_FLOW_ATTR_MASK is
missing, the entire key attributes will be displayed as exact match fields.
When OVS_FLOW_ATTR_MASK is present, but some individual key attributes do
not have matching attributes in the mask, those key attributes will be
displayed as wildcarded fields.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>