s390/mm: fix user access page-table walk code

The s390 page-table walk code, used for user copy and futex, currently
cannot handle huge pages. As far as user copy is concerned, that is
not really a problem because those functions will only be used on old
hardware that has no huge page support. But the futex code will also
use pagetable walk functions on current hardware when user space runs
in primary space mode. So, if a futex sits in a huge page, the futex
operation on it will result in a page fault loop or even data
corruption.

This patch adds the code for resolving huge page mappings in the user
access pagetable walk code on s390.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>