[POWERPC] spusched: No preemption for nosched contexts

And last but not least we need to make sure the scheduler tick never
preempts a nosched context.

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>