s390: Disable ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP

Summary

In the Linux kernel, the following vulnerability has been resolved: s390: Disable ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP As reported by Luiz Capitulino enabling HVO on s390 leads to reproducible crashes. The problem is that kernel page tables are modified without flushing corresponding TLB entries. Even if it looks like the empty flush_tlb_all() implementation on s390 is the problem, it is actually a different problem: on s390 it is not allowed to replace an active/valid page table entry with another valid page table entry without the detour over an invalid entry. A direct replacement may lead to random crashes and/or data corruption. In order to invalidate an entry special instructions have to be used (e.g. ipte or idte). Alternatively there are also special instructions available which allow to replace a valid entry with a different valid entry (e.g. crdte or cspg). Given that the HVO code currently does not provide the hooks to allow for an implementation which is compliant with the s390 architecture requirements, disable ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP again, which is basically a revert of the original patch which enabled it.