virtio_net: fix the missing of the dma cpu sync

Commit 295525e2 ("virtio_net: merge dma operations when filling
mergeable buffers") unmaps the buffer with DMA_ATTR_SKIP_CPU_SYNC when
the dma->ref is zero. We do that with DMA_ATTR_SKIP_CPU_SYNC, because we
do not want to do the sync for the entire page_frag. But that misses the
sync for the current area.

This patch does cpu sync regardless of whether the ref is zero or not.

Fixes: 295525e2 ("virtio_net: merge dma operations when filling mergeable buffers")
Reported-by: Michael Roth <michael.roth@amd.com>
Closes: http://lore.kernel.org/all/20230926130451.axgodaa6tvwqs3ut@amd.comSigned-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/net/virtio_net.c

@@ -607,16 +607,16 @@ static void virtnet_rq_unmap(struct receive_queue *rq, void *buf, u32 len)
 
 	--dma->ref;
 
-	if (dma->ref) {
-		if (dma->need_sync && len) {
-			offset = buf - (head + sizeof(*dma));
+	if (dma->need_sync && len) {
+		offset = buf - (head + sizeof(*dma));
 
-			virtqueue_dma_sync_single_range_for_cpu(rq->vq, dma->addr, offset,
-								len, DMA_FROM_DEVICE);
-		}
+		virtqueue_dma_sync_single_range_for_cpu(rq->vq, dma->addr,
+							offset, len,
+							DMA_FROM_DEVICE);
+	}
 
+	if (dma->ref)
 		return;
-	}
 
 	virtqueue_dma_unmap_single_attrs(rq->vq, dma->addr, dma->len,
 					 DMA_FROM_DEVICE, DMA_ATTR_SKIP_CPU_SYNC);