GROUP_SCHED is confirmed to cause unacceptable latencies, see:

   http://lkml.org/lkml/2008/5/2/370.

Mark it EXPERIMENTAL and default to no for now.
Signed-off-by: NParag Warudkar <parag.warudkar@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>