vhost_vdpa: fix unmap process in no-batch mode

While using the vdpa device with vIOMMU enabled
in the guest VM, when the vdpa device bind to vfio-pci and run testpmd
then system will fail to unmap.
The test process is
Load guest VM --> attach to virtio driver--> bind to vfio-pci driver
So the mapping process is
1)batched mode map to normal MR
2)batched mode unmapped the normal MR
3)unmapped all the memory
4)mapped to iommu MR

This error happened in step 3). The iotlb was freed in step 2)
and the function vhost_vdpa_process_iotlb_msg will return fail
Which causes failure.

To fix this, we will not remove the AS while the iotlb->nmaps is 0.
This will free in the vhost_vdpa_clean

Cc: stable@vger.kernel.org
Fixes: aaca8373 ("vhost-vdpa: support ASID based IOTLB API")
Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20230420151734.860168-1-lulu@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>