[S390] topology: change default

Switch default value of the kernel parameter 'topology' from off to on.
Various performance measurements have finally shown that there are no
(known) regressions anywhere.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>