[POWERPC] spusched: Catch nosched contexts in spu_deactivate

spu_deactivate should never be called for nosched contets.  Put in
a check so we can print a stacktrace and exit early in case it
happes erroneously.

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>