classifier: Add a batched miniflow lookup function.

Used in a future patch.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
dpif-netdev: Batch megaflow lookup.

Signed-off-by: Ethan Jackson <ethan@nicira.com>