blk-iocost: use local[64]_t for percpu stat

blk-iocost has been reading percpu stat counters from remote cpus which on
some archs can lead to torn reads in really rare occassions. Use local[64]_t
for those counters.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>