netdev: Dynamic per-port Flow API.

Current issues with Flow API:

* OVS calls offloading functions regardless of successful
  flow API initialization. (ex. on init_flow_api failure)
* Static initilaization of Flow API for a netdev_class forbids
  having different offloading types for different instances
  of netdev with the same netdev_class. (ex. different vports in
  'system' and 'netdev' datapaths at the same time)

Solution:

* Move Flow API from the netdev_class to netdev instance.
* Make Flow API dynamic, i.e. probe the APIs and choose the
  suitable one.

Side effects:

* Flow API providers localized as possible in their modules.
* Now we have an ability to make runtime checks. For example,
  we could check if particular device supports features we
  need, like if dpdk device supports RSS+MARK action.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Roi Dayan <roid@mellanox.com>

lib/dpdk.c

@@ -34,6 +34,7 @@
 #include "dirs.h"
 #include "fatal-signal.h"
 #include "netdev-dpdk.h"
+#include "netdev-rte-offloads.h"
 #include "openvswitch/dynamic-string.h"
 #include "openvswitch/vlog.h"
 #include "ovs-numa.h"
@@ -454,6 +455,7 @@ dpdk_init__(const struct smap *ovs_other_config)
 
     /* Finally, register the dpdk classes */
     netdev_dpdk_register();
+    netdev_dpdk_flow_api_register();
     return true;
 }