sched: Reenable interrupts in do_sched_yield()

do_sched_yield() invokes schedule() with interrupts disabled which is
not allowed. This goes back to the pre git era to commit a6efb709
("[PATCH] irqlock patch 2.5.27-H6") in the history tree.

Reenable interrupts and remove the misleading comment which "explains" it.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/87r1pt7y5c.fsf@nanos.tec.linutronix.de

kernel/sched/core.c

@@ -6094,12 +6094,8 @@ static void do_sched_yield(void)
 	schedstat_inc(rq->yld_count);
 	current->sched_class->yield_task(rq);
 
-	/*
-	 * Since we are going to call schedule() anyway, there's
-	 * no need to preempt or enable interrupts:
-	 */
 	preempt_disable();
-	rq_unlock(rq, &rf);
+	rq_unlock_irq(rq, &rf);
 	sched_preempt_enable_no_resched();
 
 	schedule();