cgroup: fix mount failure in a corner case

  # cat test.sh
  #! /bin/bash

  mount -t cgroup -o cpu xxx /cgroup
  umount /cgroup

  mount -t cgroup -o cpu,cpuacct xxx /cgroup
  umount /cgroup
  # ./test.sh
  mount: xxx already mounted or /cgroup busy
  mount: according to mtab, xxx is already mounted on /cgroup

It's because the cgroupfs_root of the first mount was under destruction
asynchronously.

Fix this by delaying and then retrying mount for this case.

v3:
- put the refcnt immediately after getting it. (Tejun)

v2:
- use percpu_ref_tryget_live() rather that introducing
  percpu_ref_alive(). (Tejun)
- adjust comment.

tj: Updated the comment a bit.

Cc: <stable@vger.kernel.org> # 3.15
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>