s390/setup: convert start and end initrd pointers to virtual

Variables initrd_start and initrd_end are expected to hold
virtual memory pointers, not physical.

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