memcg: make mem_cgroup_reparent_charges non failing

Now that pre_destroy callbacks are called from the context where neither
any task can attach the group nor any children group can be added there
is no other way to fail from mem_cgroup_pre_destroy.
mem_cgroup_pre_destroy doesn't have to take a reference to memcg's css
because all css' are marked dead already.

tj: Remove now unused local variable @cgrp from
    mem_cgroup_reparent_charges().

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: Glauber Costa <glommer@parallels.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>