s390/pgtable: Save pgste during modify_prot_start/commit

In modify_prot_start we update the pgste value but never store it back
into the original location. Lets save the calculated result, since
modify_prot_commit will use the value of the pgste.

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