virtio-ring: store DMA metadata in desc_extra for split virtqueue

For split virtqueue, we used to depend on the address, length and
flags stored in the descriptor ring for DMA unmapping. This is unsafe
for the case since the device can manipulate the behavior of virtio
driver, IOMMU drivers and swiotlb.

For safety, maintain the DMA address, DMA length, descriptor flags and
next filed of the non indirect descriptors in vring_desc_state_extra
when DMA API is used for virtio as we did for packed virtqueue and use
those metadata for performing DMA operations. Indirect descriptors
should be safe since they are using streaming mappings.

With this the descriptor ring is write only form the view of the
driver.

This slight increase the footprint of the drive but it's not noticed
through pktgen (64B) test and netperf test in the case of virtio-net.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210604055350.58753-8-jasowang@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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