s390/kasan: update kasan memory layout note

Kasan shadow memory area has been moved to the end of kernel address
space since commit 9a39abb7 ("s390/boot: simplify and fix kernel
memory layout setup"). Change kasan memory layout note accordingly.

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