KVM: s390/mm: Fix guest storage key corruption in ptep_set_access_flags

commit 0944fe3 ("s390/mm: implement software referenced bits")
triggered another paging/storage key corruption. There is an
unhandled invalid->valid pte change where we have to set the real
storage key from the pgste.
When doing paging a guest page might be swapcache or swap and when
faulted in it might be read-only and due to a parallel scan old.
An do_wp_page will make it writeable and young. Due to software
reference tracking this page was invalid and now becomes valid.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: stable@vger.kernel.org # v3.12+

arch/s390/include/asm/pgtable.h

@@ -1331,6 +1331,7 @@ static inline int ptep_set_access_flags(struct vm_area_struct *vma,
 	ptep_flush_direct(vma->vm_mm, address, ptep);
 
 	if (mm_has_pgste(vma->vm_mm)) {
+		pgste_set_key(ptep, pgste, entry, vma->vm_mm);
 		pgste = pgste_set_pte(ptep, pgste, entry);
 		pgste_set_unlock(ptep, pgste);
 	} else