cgroup: fix cgroup_taskset walking order

cgroup_taskset is used to track and iterate target tasks while
migrating a task or process and should guarantee that the first task
iterated is the task group leader if a process is being migrated.

b3dc094e ("cgroup: use css_set->mg_tasks to track target tasks
during migration") replaced flex array cgroup_taskset->tc_array with
css_set->mg_tasks list to remove process size limit and dynamic
allocation during migration; unfortunately, it incorrectly used list
operations which don't preserve order breaking the guarantee that
cgroup_taskset_first() returns the leader for a process target.

Fix it by using order preserving list operations.  Note that as
multiple src_csets may map to a single dst_cset, the iteration order
may change across cgroup_task_migrate(); however, the leader is still
guaranteed to be the first entry.

The switch to list_splice_tail_init() at the end of cgroup_migrate()
isn't strictly necessary.  Let's still do it for consistency.

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