s390/kasan: avoid confusing naming

Kasan has nothing to do with vmemmap, strip vmemmap from function names
to avoid confusing people.

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