xen/netfront: fix leaking data in shared pages

xen/netfront: fix leaking data in shared pages

This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven.
This ID is fixed in Linux Kernel version v4.9.322 by commit d1d69e0c838c2df7089357ec27000942086325c4. For more details please see the references link.