ring-buffer benchmark: Run producer/consumer threads at nice +19

The ring-buffer benchmark threads run on nice 0 by default, using
up a lot of CPU time and slowing down the system:

   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
  1024 root      20   0     0    0    0 D 95.3  0.0   4:01.67 rb_producer
  1023 root      20   0     0    0    0 R 93.5  0.0   2:54.33 rb_consumer
 21569 mingo     40   0 14852 1048  772 R  3.6  0.1   0:00.05 top
     1 root      40   0  4080  928  668 S  0.0  0.0   0:23.98 init

Renice them to +19 to make them less intrusive.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>