vhost-user-gpu: fix memory leak in vg_resource_attach_backing (CVE-2021-3544)

vhost-user-gpu: fix memory leak in vg_resource_attach_backing (CVE-2021-3544)

Check whether the 'res' has already been attach_backing to avoid
memory leak.

Fixes: CVE-2021-3544
Reported-by: Li Qiang <liq3ea@163.com>
virtio-gpu fix: 204f01b309 ("virtio-gpu: fix memory leak
in resource attach backing")

Signed-off-by: Li Qiang <liq3ea@163.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210516030403.107723-4-liq3ea@163.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit b9f79858a614d95f5de875d0ca31096eaab72c3b)
Signed-off-by: Michael Roth <michael.roth@amd.com>