powerpc/spufs: Fix possible scheduling of a context to multiple SPEs

We currently have a race when scheduling a context to a SPE -
after we have found a runnable context in spusched_tick, the same
context may have been scheduled by spu_activate().

This may result in a panic if we try to unschedule a context that has
been freed in the meantime.

This change exits spu_schedule() if the context has already been
scheduled, so we don't end up scheduling it twice.

Signed-off-by: Andre Detsch <adetsch@br.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>