tick: Don't clear idle and iowait sums on CPU down

NOHZ related per-cpu data is cleared on CPU down. This was introduced by
4b0c0f29 "tick: Cleanup NOHZ per cpu data on cpu down" which breaks
/proc/stats because the idle and iowait sums are now non-monotonic
across a CPU down/up cycle.

Fix this by not clearing the idle_sleeptime and iowait_sleeptime fields
on CPU down.

Change-Id: Ifb755e15b601c74dad81655ebb25e037dac2afd0
Signed-off-by: Sai Gurrappadi <sgurrappadi@nvidia.com>
Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Patch-mainline: linux-kernel @ 30 Apr 2014 13:18:34
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>