s390/mm: make pXd_deref() macros return a pointer

This update fixes semantics of pXd_deref macros which
are expected to return a CPU-addressable pointer.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>