powerpc/cell: Use vzalloc rather than vmalloc and memset in spu_alloc_lscsa_std

Hi,

We can get rid of a memset in
arch/powerpc/platforms/cell/spufs/lscsa_alloc.c::spu_alloc_lscsa_std() by
using vzalloc() rather than vmalloc()+memset().

Completely untested patch below since I have no hardware nor tools to
compile this.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>