blkcg: s/blkg_rwstat_sum()/blkg_rwstat_total()/

Rename blkg_rwstat_sum() to blkg_rwstat_total().  sum will be used for
summing up stats from multiple blkgs.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>