sched,psi: Convert to sched_set_fifo_low()

Because SCHED_FIFO is a broken scheduler model (see previous patches)
take away the priority field, the kernel can't possibly make an
informed decision.

Effectively no change.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>

kernel/sched/psi.c

@@ -616,11 +616,8 @@ static void psi_poll_work(struct psi_group *group)
 static int psi_poll_worker(void *data)
 {
 	struct psi_group *group = (struct psi_group *)data;
-	struct sched_param param = {
-		.sched_priority = 1,
-	};
 
-	sched_setscheduler_nocheck(current, SCHED_FIFO, &param);
+	sched_set_fifo_low(current);
 
 	while (true) {
 		wait_event_interruptible(group->poll_wait,