s390/mm,kvm: fix software dirty bits vs. kvm for old machines

For machines without enhanced supression on protection the software
dirty bit code forces the pte dirty bit and clears the page protection
bit in pgste_set_pte. This is done for all pte types, the check for
present ptes is missing. As a result swap ptes and other not-present
ptes can get corrupted.
Add a check for the _PAGE_PRESENT bit to pgste_set_pte before modifying
the pte value.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/pgtable.h

@@ -748,7 +748,9 @@ static inline void pgste_set_key(pte_t *ptep, pgste_t pgste, pte_t entry)
 
 static inline void pgste_set_pte(pte_t *ptep, pte_t entry)
 {
-	if (!MACHINE_HAS_ESOP && (pte_val(entry) & _PAGE_WRITE)) {
+	if (!MACHINE_HAS_ESOP &&
+	    (pte_val(entry) & _PAGE_PRESENT) &&
+	    (pte_val(entry) & _PAGE_WRITE)) {
 		/*
 		 * Without enhanced suppression-on-protection force
 		 * the dirty bit on for all writable ptes.