[S390] No panic for failed reboot

If reboot fails (e.g. because wrong devno has been specified by the user),
we should just stop all cpus, but should not trigger a kernel panic.

Signed-off-by: Michael Holzheu <holzheu@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>