virtio_pci_modern: __force cast the notify mapping

When switching virtio_pci_modern to use a helper for mappings we lost an
__iomem tag. We should restore it.

However, virtio_pci_modern is playing tricks by hiding an iomem pointer
in a regular vq->priv pointer. Which is okay as long as it's
all contained within a single file, but we need to __force cast
the value otherwise we'll get sparse warnings.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 7dca6c0e ("virtio-pci library: switch to use vp_modern_map_vq_notify()")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>