[POWERPC] spusched: Fix runqueue corruption

spu_activate can be called from multiple threads at the same time on
behalf of the same spu context.  We need to make sure to only add it
once to avoid runqueue corruption.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>