[POWERPC] spufs: block fault handlers in spu_acquire_runnable

This change disables the logic that faults-in spu contexts under the
covers from the page fault handler.  When a fault requires a runnable
context, the handler will block until the context is scheduled by
other means.

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>