spufs: make signal-notification files readonly for NOSCHED contexts

Reading from the signal{1,2} files requires a spu_acquire_saved, so make these
files write-only for contexts created with SPU_CREATE_NOSCHED.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Acked-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>