s390/mm/ptdump: Fix handling of identity mapping area

Since virtual and real addresses are not the same anymore the
assumption that the kernel image is contained within the identity
mapping is also not true anymore.

Fix this by adding two explicit areas and at the correct locations: one
for the 8kb lowcore area, and one for the identity mapping.

Fixes: c98d2eca ("s390/mm: Uncouple physical vs virtual address spaces")
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>