dpif-netdev: Introduce netdev_flow_key_* functions

dpif-netdev: Introduce netdev_flow_key_* functions

netdev_flow_key is a miniflow with the following constraints:

1) It is used only inside dpif-netdev.c.
2) It always has inline values.
3) It contains only miniflows created by miniflow_extract().

Therefore, by using these new functions instead of the miniflow_*
ones, we get the following (performance related) benefits:

- Because of (1) the functions can be inlined.
- Because of (2) and (3) the netdev_flow_key can be treated as POD.
  Specifically, because of (3), we can do comparisons with memcmp,
  since if the map is different the miniflow must be different.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>