introduce virtqueue_get_buf_ctx_dma() to collect the dma info when
get buf from virtio core for premapped mode.

If the virtio queue is premapped mode, the virtio-net send buf may
have many desc. Every desc dma address need to be unmap. So here we
introduce a new helper to collect the dma address of the buffer from
the virtio core.

Because the BAD_RING is called (that may set vq->broken), so
the relative "const" of vq is removed.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 drivers/virtio/virtio_ring.c | 174 +++++++++++++++++++++++++----------
 include/linux/virtio.h       |  16 ++++
 2 files changed, 142 insertions(+), 48 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 51d8f3299c10..1374b3fd447c 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -362,6 +362,45 @@ static struct device *vring_dma_dev(const struct vring_virtqueue *vq)
        return vq->dma_dev;
 }

+/*
+ *     use_dma_api premapped -> do_unmap
+ *  1. false       false        false
+ *  2. true        false        true
+ *  3. true        true         false
+ *
+ * Only #3, we should return the DMA info to the driver.