sched: fix regression with sched yield

Balbir Singh reported:

> 1:mon> t
> [c0000000e7677da0] c000000000067de0 .sys_sched_yield+0x6c/0xbc
> [c0000000e7677e30] c000000000008748 syscall_exit+0x0/0x40
> --- Exception: c01 (System Call) at 00000400001d09e4
> SP (4000664cb10) is in userspace
> 1:mon> r
> cpu 0x1: Vector: 300 (Data Access) at [c0000000e7677aa0]
> pc: c000000000068e50: .yield_task_fair+0x94/0xc4
> lr: c000000000067de0: .sys_sched_yield+0x6c/0xbc

the check that should have avoided that is:

/*
* Are we the only task in the tree?
*/
if (unlikely(rq->load.weight == curr->se.load.weight))
return;

But I guess that overlooks rt tasks, they also increase the load.
So I guess something like this ought to fix it..

Signed-off-by: Ingo Molnar <[e-mail]>

kernel/sched_fair.c

···
 	/*
 	 * Already in the rightmost position?
 	 */
-	if (unlikely(rightmost->vruntime < se->vruntime))
+	if (unlikely(!rightmost || rightmost->vruntime < se->vruntime))
 		return;
 
 	/*