ofproto-dpif: Make sure one-packet flows have zero duration.

A Nicira internal build recently failed the "ofproto-dpif - NetFlow flow
expiration" test because of the following difference in output:

     header: v5, seq 0, engine 2,1
    -rec: [...], 1 pkts, 60 bytes, ICMP 8:0, time <moment>
    +rec: [...], 1 pkts, 60 bytes, ICMP 8:0, time <range>

Looking at the actual output, it is:
    rec: 192.168.0.1 > 192.168.0.2, if 1 > 65535, 1 pkts, 60 bytes,
    ICMP 8:0, time 8...9

That is, a one-packet flow was shown to have more than a momentary
duration, which doesn't make sense.

This commit fixes the problem by making sure that creating a facet and then
its initial subfacet only checks the current time once.

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