UPSTREAM: cgroup: remove extra cgroup_migrate_finish() call

The callers of cgroup_migrate_prepare_dst() correctly call
cgroup_migrate_finish() for success and failure cases both. No need to
call it in cgroup_migrate_prepare_dst() in failure case.

Signed-off-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Change-Id: I785d7ab70a42b1b79aea9852bb14ba5abefcaa9b
(cherry picked from commit d6e486ee0ef2f99a4069d9186e53dac61b28cb3c)
Bug: 154548692
Signed-off-by: Marco Ballesio <balejs@google.com>