sched/walt: Use __GFP_NOFAIL flag for curr/prev window CPU arrays

The system is killed when the memory allocation is failed for
the curr/prev window CPU arrays. Since these are small allocations
and they never fail except in scenarios like

(1) Testing with random memory failures induced
(2) When a fatal signal is pending for the task which is doing
allocations.

Instead of killing the system, use __GFP_NOFAIL flag.

Change-Id: If284814db5a6504b2f039053856ff30c2842808b
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>