[PATCH] powerpc: cell: use kzalloc in alloc_spu_context()

Use kzalloc when allocating a new spu context, rather than kmalloc +
zeroing.

Booted & tested on cell.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>