KVM: s390: pv: fix index value of replaced ASCE

In the Linux kernel, the following vulnerability has been resolved: KVM: s390: pv: fix index value of replaced ASCE The index field of the struct page corresponding to a guest ASCE should be 0. When replacing the ASCE in s390_replace_asce(), the index of the new ASCE should also be set to 0. Having the wrong index might lead to the wrong addresses being passed around when notifying pte invalidations, and eventually to validity intercepts (VM crash) if the prefix gets unmapped and the notifier gets called with the wrong address.