sched: window-stats: Retain idle thread's mark_start

init_idle() is called on a cpu's idle-thread once at bootup and
subsequently everytime the cpu is hot-added. Since init_idle() calls
__sched_fork(), we end up blowing idle thread's ravg.mark_start value.
As a result we will fail to accurately maintain cpu's
curr/prev_runnable_sum counters. Below example illustrates such a
failure:

CS = curr_runnable_sum, PS = prev_runnable_sum

t0 -> New window starts for CPU2
<after some_task_activity> CS = X, PS = Y
t1 -> <cpu2 is hot-removed. idle_task start's running on cpu2>
      At this time, cpu2_idle_thread.ravg.mark_start = t1

t1 -> t0 + W. One window elapses. CPU2 still hot-removed. We
	defer swapping CS and PS until some future task event occurs

t2 -> CPU2 hot-added.  _cpu_up()->idle_thread_get()->init_idle()
	->__sched_fork() results in cpu2_idle_thread.ravg.mark_start = 0

t3 -> Some task wakes on cpu2. Since mark_start = 0, we don't swap CS
	and PS => which is a BUG!

Fix this by retaining idle task's original mark_start value during
init_idle() call.

Change-Id: I4ac9bfe3a58fb5da8a6c7bc378c79d9930d17942
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>