sched,rcutorture: 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.

Cc: paulmck@kernel.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcutorture.c

@@ -889,13 +889,11 @@ static int rcu_torture_boost(void *arg)
 	unsigned long endtime;
 	unsigned long oldstarttime;
 	struct rcu_boost_inflight rbi = { .inflight = 0 };
-	struct sched_param sp;
 
 	VERBOSE_TOROUT_STRING("rcu_torture_boost started");
 
 	/* Set real-time priority. */
-	sp.sched_priority = 1;
-	if (sched_setscheduler(current, SCHED_FIFO, &sp) < 0) {
+	if (sched_set_fifo_low(current) < 0) {
 		VERBOSE_TOROUT_STRING("rcu_torture_boost RT prio failed!");
 		n_rcu_torture_boost_rterror++;
 	}