vfio iommu: Cache pgsize_bitmap in struct vfio_iommu

Calculate and cache pgsize_bitmap when iommu->domain_list is updated
and iommu->external_domain is set for mdev device.
Add iommu->lock protection when cached pgsize_bitmap is accessed.

Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
Reviewed-by: Neo Jia <cjia@nvidia.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>