dpif: Add support for OVS_ACTION_ATTR_CT_CLEAR

This supports using the ct_clear action in the kernel datapath. To
preserve compatibility with current ct_clear behavior on old kernels, we
only pass this action down to the datapath if a probe reveals the
datapath actually supports it.

Signed-off-by: Eric Garver <e@erig.me>
Acked-by: William Tu <u9012063@gmail.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Justin Pettit <jpettit@ovn.org>

lib/dpif-netdev.c

@@ -5762,6 +5762,7 @@ dp_execute_cb(void *aux_, struct dp_packet_batch *packets_,
     case OVS_ACTION_ATTR_CLONE:
     case OVS_ACTION_ATTR_PUSH_NSH:
     case OVS_ACTION_ATTR_POP_NSH:
+    case OVS_ACTION_ATTR_CT_CLEAR:
     case __OVS_ACTION_ATTR_MAX:
         OVS_NOT_REACHED();
     }