vdpa_sim: use kvmalloc to allocate vdpasim->buffer

The next patch will make the buffer size configurable from each
device.
Since the buffer could be larger than a page, we use kvmalloc()
instead of kmalloc().
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-16-sgarzare@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vdpa/vdpa_sim/vdpa_sim.c

@@ -418,7 +418,7 @@ static struct vdpasim *vdpasim_create(struct vdpasim_dev_attr *dev_attr)
 	if (!vdpasim->iommu)
 		goto err_iommu;
 
-	vdpasim->buffer = kmalloc(PAGE_SIZE, GFP_KERNEL);
+	vdpasim->buffer = kvmalloc(PAGE_SIZE, GFP_KERNEL);
 	if (!vdpasim->buffer)
 		goto err_iommu;
 
@@ -707,7 +707,7 @@ static void vdpasim_free(struct vdpa_device *vdpa)
 	struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
 
 	cancel_work_sync(&vdpasim->work);
-	kfree(vdpasim->buffer);
+	kvfree(vdpasim->buffer);
 	if (vdpasim->iommu)
 		vhost_iotlb_free(vdpasim->iommu);
 	kfree(vdpasim->vqs);