netflow: Correctly track flow creation time.

'created' is supposed to be the time the flow was created, but it was
getting reset to zero on every expiration, causing the flow start time to
be wonky after the first active expiration on a flow.

Reported-by: Lior Neudorfer <lior@guardicore.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>