[POWERPC] spusched: Fix null pointer dereference in find_victim

find_victim can dereference a NULL pointer when iterating over the list
of victim spus because list_mutex only guarantees spu->ct to be stable,
but of course not to be non-NULL.

Also fix find_victim to not call spu_unbind_context without list_mutex
because that violates the above guarantee.

Signed-off-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Jeremy Kerr <[e-mail]>
Signed-off-by: Paul Mackerras <[e-mail]>

arch/powerpc/platforms/cell/spufs/sched.c

···
 		list_for_each_entry(spu, &cbe_spu_info[node].spus, cbe_list) {
 			struct spu_context *tmp = spu->ctx;
 
-			if (tmp->prio > ctx->prio &&
+			if (tmp && tmp->prio > ctx->prio &&
 			    (!victim || tmp->prio > victim->prio))
 				victim = spu->ctx;
 		}
···
 
 			mutex_lock(&cbe_spu_info[node].list_mutex);
 			cbe_spu_info[node].nr_active--;
+			spu_unbind_context(spu, victim);
 			mutex_unlock(&cbe_spu_info[node].list_mutex);
 
-			spu_unbind_context(spu, victim);
 			victim->stats.invol_ctx_switch++;
 			spu->stats.invol_ctx_switch++;
 			mutex_unlock(&victim->state_mutex);