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>