sched: Fix priority inversion of cookied task with sibling

The rationale is as follows. In the core-wide pick logic, even if
need_sync == false, we need to go look at other CPUs (non-local CPUs)
to see if they could be running RT.

Say the RQs in a particular core look like this:

Let CFS1 and CFS2 be 2 tagged CFS tags.
Let RT1 be an untagged RT task.

	rq0		rq1
	CFS1 (tagged)	RT1 (no tag)
	CFS2 (tagged)

Say schedule() runs on rq0. Now, it will enter the above loop and
pick_task(RT) will return NULL for 'p'. It will enter the above if()
block and see that need_sync == false and will skip RT entirely.

The end result of the selection will be (say prio(CFS1) > prio(CFS2)):

	rq0             rq1
	CFS1            IDLE

When it should have selected:

	rq0             rq1
	IDLE            RT

Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Don Hiatt <dhiatt@digitalocean.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Tested-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210422123308.678425748@infradead.org