spufs: Convert nopfn to fault

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>