s390x/helper: Fixed real-to-absolute address translation

s390x/helper: Fixed real-to-absolute address translation

The real-to-absolute address translation in mmu_translate() was
missing the second part for translating the page at the prefix
address back to the 0 page. And while we're at it, also moved the
code into a separate helper function since this might come in
handy for other parts of the code, too.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>