vhost_vdpa: switch to vmemdup_user()

Replace opencoded alloc and copy with vmemdup_user()

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Link: https://lore.kernel.org/r/1605057288-60400-1-git-send-email-tiantao6@hisilicon.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>