Pinned pages are not properly accounted particularly when
mapping error occurs on IOTLB update. Clean up dangling
pinned pages for the error path.

The memory usage for bookkeeping pinned pages is reverted
to what it was before: only one single free page is needed.
This helps reduce the host memory demand for VM with a large
amount of memory, or in the situation where host is running
short of free memory.

Fixes: 4c8cf318 ("vhost: introduce vDPA-based backend")
Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Link: https://lore.kernel.org/r/1604618793-4681-1-git-send-email-si-wei.liu@oracle.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>