ofproto-dpif: Require an in_port when tracing datapath flows.

ofproto-dpif: Require an in_port when tracing datapath flows.

All datapath flows should have an in_port, so it doesn't make a lot
of sense to allow omitting it when tracing.  If a user wants to
trace a flow which has no in_port, they can use the OpenFlow syntax
which doesn't go through ofproto_receive().

Signed-off-by: Ethan Jackson <ethan@nicira.com>