sched,tracing: Convert to sched_set_fifo()

One module user of sched_setscheduler() was overlooked and is
obviously causing build failures.

Convert ring_buffer_benchmark to use sched_set_fifo_low() when fifo==1
and sched_set_fifo() when fifo==2. This is a bit of an abuse, but it
makes the thing 'work' again.

Specifically, it enables all combinations that were previously
possible:

  producer higher than consumer
  consumer higher than producer

Fixes: 616d91b6 ("sched: Remove sched_setscheduler*() EXPORTs")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://lkml.kernel.org/r/20200720214918.GM5523@worktop.programming.kicks-ass.net