dpif-netdev: Add clone action

Add support for userspace datapath clone action.  The clone action
provides an action envelope to enclose an action list.
For example, with actions A, B, C and D,  and an action list:
      A, clone(B, C), D

The clone action will ensure that:

- D will see the same packet, and any meta states, such as flow, as
  action B.

- D will be executed regardless whether B, or C drops a packet. They
  can only drop a clone.

- When B drops a packet, clone will skip all remaining actions
  within the clone envelope. This feature is useful when we add
  meter action later:  The meter action can be implemented as a
  simple action without its own envolop (unlike the sample action).
  When necessary, the flow translation layer can enclose a meter action
  in clone.

The clone action is very similar with the OpenFlow clone action.
This is by design to simplify vswitchd flow translation logic.

Without datapath clone, vswitchd simulate the effect by inserting
datapath actions to "undo" clone actions. The above flow will be
translated into   A, B, C, -C, -B, D.

However, there are two issues:
- The resulting datapath action list may be longer without using
  clone.

- Some actions, such as NAT may not be possible to reverse.

This patch implements clone() simply with packet copy. The performance
can be improved with later patches, for example, to delay or avoid
packet copy if possible.  It seems datapath should have enough context
to carry out such optimization without the userspace context.

Signed-off-by: Andy Zhou <azhou@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>

lib/dpif-netdev.c

@@ -4731,6 +4731,7 @@ dp_execute_cb(void *aux_, struct dp_packet_batch *packets_,
     case OVS_ACTION_ATTR_HASH:
     case OVS_ACTION_ATTR_UNSPEC:
     case OVS_ACTION_ATTR_TRUNC:
+    case OVS_ACTION_ATTR_CLONE:
     case __OVS_ACTION_ATTR_MAX:
         OVS_NOT_REACHED();
     }