cgroup: make cgroup[_taskset]_migrate() take cgroup_root instead of cgroup

cgroup: make cgroup[_taskset]_migrate() take cgroup_root instead of cgroup

On the default hierarchy, a migration can be multi-source and/or
multi-destination.  cgroup_taskest_migrate() used to incorrectly
assume single destination cgroup but the bug has been fixed by
5206feaf35c9 ("cgroup: fix handling of multi-destination migration
from subtree_control enabling").

Since the commit, @dst_cgrp to cgroup[_taskset]_migrate() is only used
to determine which subsystems are affected or which cgroup_root the
migration is taking place in.  As such, @dst_cgrp is misleading.  This
patch replaces @dst_cgrp with @root.

Signed-off-by: Tejun Heo <tj@kernel.org>