vhost-net: cleanup macros for DMA status tracking

Better document macros for DMA tracking. Add an
explicit one for DMA in progress instead of
relying on user supplying len != 1.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 072cbba..f80ae5f 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -237,7 +237,8 @@
 			} else {
 				struct ubuf_info *ubuf = &vq->ubuf_info[head];
 
-				vq->heads[vq->upend_idx].len = len;
+				vq->heads[vq->upend_idx].len =
+					VHOST_DMA_IN_PROGRESS;
 				ubuf->callback = vhost_zerocopy_callback;
 				ubuf->ctx = vq->ubufs;
 				ubuf->desc = vq->upend_idx;