sched: prevent task migration while governor queries CPUs' load

At present, governor retrieves each CPUs' load sequentially.  In this
way, there is chance of race between governor's CPU load query and task
migration that would result in reporting of lesser CPUs' load than actual.

For example,
CPU0 load = 30%.  CPU1 load = 50%.
Governor                               Load balancer
- sched_get_busy(cpu 0) = 30%.
                                       - A task 'p' migrated from CPU 1 to
                                         CPU 0.  p->ravg->prev_window = 50.
                                         Now CPU 0's load = 80%,
                                         CPU 1's load = 0%.
- sched_get_busy(cpu 1) = 0%
  50% of load from CPU 1 to 0 never
  accounted.

Fix such issues by introducing a new API sched_get_cpus_busy() which
makes for governor to be able to get set of CPUs' load.  The loads set
internally constructed with blocking load balancer to ensure migration
cannot occur in the meantime.

Change-Id: I4fa4dd1195eff26aa603829aca2054871521495e
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>