s390/kasan: avoid vdso instrumentation

vdso is mapped into user space processes, which won't have kasan
shodow mapped.

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>