sched: switch sched_features to seqfile

Impact: cleanup

So handling of sched_features read is simplified.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>