s390x/mmu: Fix the check for the real-space designation bit

The real-space designation bits live in the ASCEs, not in the table entries,
so the check must be done before we start walking the MMU table.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
