vduse: Use memcpy_{to,from}_page() in do_bounce()

kmap_atomic() is being deprecated in favor of kmap_local_page().

The use of kmap_atomic() in do_bounce() is all thread local therefore
kmap_local_page() is a sufficient replacement.

Convert to kmap_local_page() but, instead of open coding it,
use the helpers memcpy_to_page() and memcpy_from_page().

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Message-Id: <20220803045523.23851-3-xieyongji@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>