cgroup: export cgrp_dfl_root

While cgroup subsystems can't be modules, blkcg supports dynamically
loadable policies which interact with cgroup core.  Export
cgrp_dfl_root so that cgroup_on_dfl() can be used in those modules.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>