s390/mm: Prevent lowcore vs identity mapping overlap

s390/mm: Prevent lowcore vs identity mapping overlap

The identity mapping position in virtual memory is randomized
together with the kernel mapping. That position can never
overlap with the lowcore even when the lowcore is relocated.

Prevent overlapping with the lowcore to allow independent
positioning of the identity mapping. With the current value
of the alternative lowcore address of 0x70000 the overlap
could happen in case the identity mapping is placed at zero.

This is a prerequisite for uncoupling of randomization base
of kernel image and identity mapping in virtual memory.

Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>