vduse: prevent uninitialized memory accesses

If the VDUSE application provides a smaller config space
than the driver expects, the driver may use uninitialized
memory from the stack.

This patch prevents it by initializing the buffer passed by
the driver to store the config value.

This fix addresses CVE-2022-2308.

Cc: stable@vger.kernel.org # v5.15+
Fixes: c8a6153b

 ("vduse: Introduce VDUSE - vDPA Device in Userspace")
Reviewed-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Message-Id: <20220831154923.97809-1-maxime.coquelin@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>