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>