dpif-netdev: Prefetch next packet before miniflow_extract().

dpif-netdev: Prefetch next packet before miniflow_extract().

It appears that miniflow_extract() in emc_processing() spends a lot of
cycles waiting for the packet's data to be read.

Prefetching the next packet's data while parsing removes this delay.
For a single flow pipeline the throughput improves by ~10%.  With a
more realistic pipeline the change has a much smaller effect (~0.5%
improvement)

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>