vduse: Fix returning wrong type in vduse_domain_alloc_iova()

vduse: Fix returning wrong type in vduse_domain_alloc_iova()

This fixes the following smatch warnings:

drivers/vdpa/vdpa_user/iova_domain.c:305 vduse_domain_alloc_iova() warn: should 'iova_pfn << shift' be a 64 bit type?

Fixes: 5d08386a2ab1 ("vduse: Implement an MMU-based software IOTLB")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20220121083940.102-1-xieyongji@bytedance.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>