blkcg: move blkg_for_each_descendant_pre() to block/blk-cgroup.h

blk-throttle hierarchy support will make use of it.  Move
blkg_for_each_descendant_pre() from block/blk-cgroup.c to
block/blk-cgroup.h.

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