s390/vmem: fix empty page tables cleanup under KASAN

Commit b9ff8100 ("s390/vmem: cleanup empty page tables") introduced
empty page tables cleanup in vmem code, but when the kernel is built
with KASAN enabled the code has no effect due to wrong KASAN shadow
memory intersection condition, which effectively ignores any memory
range below KASAN shadow. Fix intersection condition to make code
work as anticipated.

Fixes: b9ff8100 ("s390/vmem: cleanup empty page tables")
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>