[POWERPC] spu sched: ensure preempted threads are put back on the runqueue, part2

To not lose a spu thread we need to make sure it always gets put back
on the runqueue.  In find_victim aswell as in the scheduler tick as done
in the previous patch.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>