s390/pgtable: add pgste_get helper

ptep_modify_prot_start uses the pgste_set helper to store the pgste,
while ptep_modify_prot_commit uses its own pointer magic to retrieve
the value again. Add the pgste_get help function to keep things
symmetrical and improve readability.

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