Separate out cfqg_stats_reset() which takes struct cfqg_stats * from
cfq_pd_reset_stats() and move the latter to where other pd methods are
defined.  cfqg_stats_reset() will be used to implement hierarchical
stats.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>