virtio_pci_modern: __force cast the notify mapping

stable inclusion
from stable-v5.13
commit 0f8a0b0b
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5WXCZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0f8a0b0b095fd9b301523c0f78686f5ac6fda564


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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>
Signed-off-by: Pengyuan Zhao <zhaopengyuan@hisilicon.com>