util: kernel: Fix to return valid result on getting cpu stat

util: kernel: Fix to return valid result on getting cpu stat

In the case of the target which supports cpu hotplug, the number of
possible online cpus can be changed and thus __get_cpu_stat can be
failed for offline cpus. At worst case if the last __get_cpu_stat is
resulted in negative, kernel_get_per_cpu_stat can return error even
though it is not actual failure. To prevent this situation, this
ignores the case that __get_cpu_stat returns failure.

Change-Id: I5d33e740b2451e7c644b407c1f92785a1e570585
Signed-off-by: Dongwoo Lee <dwoo08.lee@samsung.com>