ofproto-dpif: Close race between processing packet_ins and checking seqno.

If a packet-in were to be queued, and the sequence number changed, after
grabbing the list of packet-ins, then the existing code could have gone to
sleep until something happened.  By grabbing the sequence number before
the list of packet-ins, we avoid this race.

Found by inspection.

Signed-off-by: Ben Pfaff <blp@nicira.com>