sched: s/sched_latency/sched_min_granularity

runtime limit and wakeup granularity used to be a function of
granularity and that was incorrect changed to sched_latency.

Fix this to make wakeup granularity a function of min-granularity,
and the runtime limit equal to latency.

Signed-off-by: Ingo Molnar <mingo@elte.hu>