[POWERPC] spufs: Always map local store non-guarded

When fixing spufs to map the 'mem' file backing store cacheable,
I incorrectly set the physical mapping to use both cache-inhibited
and guarded mapping, which resulted in a serious performance
degradation.

Debugged-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>