sched: use instant load for burst wake up

If many tasks sleep long time, their runnable load are zero. And if they
are waked up bursty, too light runnable load causes big imbalance among
CPU. So such benchmark, like aim9 drop 5~7%.

With this patch the losing is covered, and even is slight better.

Signed-off-by: Alex Shi <alex.shi@intel.com>