[POWERPC] spufs scheduler: Fix wakeup races

Fix the race between checking for contexts on the runqueue and actually
waking them in spu_deactive and spu_yield.

The guts of spu_reschedule are split into a new helper called
grab_runnable_context which shows if there is a runnable thread below
a specified priority and if yes removes if from the runqueue and uses
it.  This function is used by the new __spu_deactivate hepler shared
by preemption and spu_yield to grab a new context before deactivating
a specified priority and if yes removes if from the runqueue and uses
it.  This function is used by the new __spu_deactivate hepler shared
by preemption and spu_yield to grab a new context before deactivating
the old one.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>