cgroup: cosmetic updates to rebind_subsystems()

* Use local variables @scgrp and @dcgrp for @src_root->cgrp and
  @dst_root->cgrp respectively.

* Use initializers to set @src_root and @css in the inner bind loop.

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