s390/kasan: avoid unnecessary moving of vmemmap

Currently vmemmap area is unconditionally moved beyond Kasan shadow
memory. When Kasan is not enabled vmemmap area position is calculated
in setup_memory_end() and depends on limiting factors like ultravisor
secure storage limit. Try to follow the same logic with Kasan enabled
as well and avoid unnecessary vmemmap area position changes unless it
really intersects with Kasan shadow.

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