Process DiscardingFlowOp correctly in FastDatapath send()

The nil FlowOp check in collectFlowOps was not addressed with the
introduction of DiscardingFlowOp.  This meant that DiscardingFlowOps
produced in the router core would get handled by fastdp as if they were
foreign FlowOps, preventing flow creation and so leading to unwarranted
ODP misses.

To fix this, collectFlowOps cannot simply eliminate FlowOps for which
Discards() is true, because it's true for odpFlowKeyFlowOp.  So the
Discards() case has to be handled in FastDatapath send().

router/fastdp.go

@@ -775,10 +775,10 @@ func (fwd *fastDatapathForwarder) Forward(key ForwardPacketKey) FlowOp {
 	defer fwd.lock.RUnlock()
 
 	if fwd.remoteAddr == nil {
-		// Returning nil would discard the packet, but also
-		// result in a flow rule, which we would have to
-		// invalidate when we learn the remote IP.  So for
-		// now, just prevent flows.
+		// Returning a DiscardingFlowOp would discard the
+		// packet, but also result in a flow rule, which we
+		// would have to invalidate when we learn the remote
+		// IP.  So for now, just prevent flows.
 		return vetoFlowCreationFlowOp{}
 	}
 
@@ -1086,6 +1086,10 @@ func (fastdp *FastDatapath) send(fops FlowOp, frame []byte, lock *fastDatapathLo
 		case vetoFlowCreationFlowOp:
 			createFlow = false
 		default:
+			if xfop.Discards() {
+				continue
+			}
+
 			// A foreign FlowOp (e.g. a sleeve forwarding
 			// FlowOp), so send the packet through the
 			// FlowOp interface, decoding the packet

router/flow.go

@@ -84,15 +84,12 @@ func (mfop *MultiFlowOp) Discards() bool {
 	return true
 }
 
+// Flatten out a FlowOp to eliminate any MultiFlowOps
 func FlattenFlowOp(fop FlowOp) []FlowOp {
 	return collectFlowOps(nil, fop)
 }
 
 func collectFlowOps(into []FlowOp, fop FlowOp) []FlowOp {
-	if fop == nil {
-		return into
-	}
-
 	if mfop, ok := fop.(*MultiFlowOp); ok {
 		for _, op := range mfop.ops {
 			into = collectFlowOps(into, op)