vhost_vdpa: return -EFAULT if copy_to_user() fails

The copy_to_user() function returns the number of bytes remaining to be
copied but this should return -EFAULT to the user.

Fixes: 1b48dc03

 ("vhost: vdpa: report iova range")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/X8c32z5EtDsMyyIL@mwanda
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>