sched/fair: Do not announce throttled next buddy in dequeue_task_fair()

sched/fair: Do not announce throttled next buddy in dequeue_task_fair()

Hierarchy could be already throttled at this point. Throttled next
buddy could trigger a NULL pointer dereference in pick_next_task_fair().

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ben Segall <bsegall@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/146608183552.21905.15924473394414832071.stgit@buzz
Signed-off-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit 754bd598be9bbc953bc709a9e8ed7f3188bfb9d7)

Orabug: 27787518

Signed-off-by: Gayatri Vasudevan <gayatri.vasudevan@oracle.com>
Reviewed-by: Mridula Shastry <mridula.c.shastry@oracle.com>