virtio_net: Use proper frame length in receive path.

Previously the frame length was set to the allocation size of the buffer
itself, which is constant. Use the length returned on dequeing instead,
which tells how much of the buffer was actually filled.

Fixes overly long frames that lead to various problems along the receive
path.

src/add-ons/kernel/drivers/network/virtio/virtio_net.cpp

@@ -46,6 +46,7 @@ struct BufInfo : DoublyLinkedListLinkImpl<BufInfo> {
 	struct virtio_net_hdr*	hdr;
 	physical_entry			entry;
 	physical_entry			hdrEntry;
+	uint32					rxUsedLength;
 };
 
 
@@ -549,17 +550,20 @@ virtio_net_read(void* cookie, off_t pos, void* buffer, size_t* _length)
 
 		mutex_lock(&info->rxLock);
 		while (info->rxDone != -1) {
+			uint32 usedLength = 0;
 			BufInfo* buf = (BufInfo*)info->virtio->queue_dequeue(
-				info->rxQueues[0], NULL);
+				info->rxQueues[0], &usedLength);
 			if (buf == NULL)
 				break;
+
+			buf->rxUsedLength = usedLength;
 			info->rxFullList.Add(buf);
 		}
 		TRACE("virtio_net_read: finished waiting\n");
 	}
 
 	BufInfo* buf = info->rxFullList.RemoveHead();
-	*_length = MIN(buf->entry.size, *_length);
+	*_length = MIN(buf->rxUsedLength, *_length);
 	memcpy(buffer, buf->buffer, *_length);
 	virtio_net_rx_enqueue_buf(info, buf);
 	mutex_unlock(&info->rxLock);