flow: Rearrange struct flow for better megaflows.

Since the dp_hash will often be a hash of the 5 tuple, it makes sense
to put it with the L4 header so it hits in the last classifier lookup
stage.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>