vop: sparse warning fixup

vop_dc_to_vdev dropped an __iomem tag on its argument,
causing a sparse warning. Fix it up.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/r/20200710113447.427927-1-mst@redhat.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>