[POWERPC] spufs: reacquire LS pointer in spu_process_callback

During spu_process callback, we release then acquire the SPU, but keep a
pointer to the local store memory. Since the context may have been
scheduled out during the callback, the ls pointer may become invalid.

This change reacquires the pointer to the context local store after
spu_acquire()-ing, so that it isn't invalidated by a context switch.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>