Fix missing mutex_spin_on_owner function. Should fix i915 problems.

Index: linux-2.6.31-bfs/kernel/sched_bfs.c
===================================================================
--- linux-2.6.31-bfs.orig/kernel/sched_bfs.c	2009-10-01 20:21:17.637656687 +1000
+++ linux-2.6.31-bfs/kernel/sched_bfs.c	2009-10-01 20:22:03.457656220 +1000
@@ -2436,7 +2436,54 @@ EXPORT_SYMBOL(schedule);
 #ifdef CONFIG_SMP
 int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
 {
-	return 0;
+	unsigned int cpu;
+	struct rq *rq;
+
+#ifdef CONFIG_DEBUG_PAGEALLOC
+	/*
+	 * Need to access the cpu field knowing that
+	 * DEBUG_PAGEALLOC could have unmapped it if
+	 * the mutex owner just released it and exited.
+	 */
+	if (probe_kernel_address(&owner->cpu, cpu))
+		goto out;
+#else
+	cpu = owner->cpu;
+#endif
+
+	/*
+	 * Even if the access succeeded (likely case),
+	 * the cpu field may no longer be valid.
+	 */
+	if (cpu >= nr_cpumask_bits)
+		goto out;
+
+	/*
+	 * We need to validate that we can do a
+	 * get_cpu() and that we have the percpu area.
+	 */
+	if (!cpu_online(cpu))
+		goto out;
+
+	rq = cpu_rq(cpu);
+
+	for (;;) {
+		/*
+		 * Owner changed, break to re-assess state.
+		 */
+		if (lock->owner != owner)
+			break;
+
+		/*
+		 * Is that owner really running on that cpu?
+		 */
+		if (task_thread_info(rq->curr) != owner || need_resched())
+			return 0;
+
+		cpu_relax();
+	}
+out:
+	return 1;
 }
 #endif