Currently, creating and removing cgroup files in the root directory
are handled separately from the actual subsystem binding and unbinding
which happens in rebind_subsystems().  Also, rebind_subsystems() users
aren't handling file creation errors properly.  Let's integrate
top_cgroup file handling into rebind_subsystems() so that it's simpler
to use and everyone handles file creation errors correctly.

* On a successful return, rebind_subsystems() is guaranteed to have
  created all files of the new subsystems and deleted the ones
  belonging to the removed subsystems.  After a failure, no file is
  created or removed.

* cgroup_remount() no longer needs to make explicit populate/clear
  calls as it's all handled by rebind_subsystems(), and it gets proper
  error handling automatically.

* cgroup_mount() has been updated such that the root dentry and cgroup
  are linked before rebind_subsystems().  Also, the init_cred dancing
  and base file handling are moved right above rebind_subsystems()
  call and proper error handling for the base files is added.  While
  at it, add a comment explaining what's going on with the cred thing.

* cgroup_kill_sb() calls rebind_subsystems() to unbind all subsystems
  which now implies removing all subsystem files which requires the
  directory's i_mutex.  Grab it.  This means that files on the root
  cgroup are removed earlier - they used to be deleted from generic
  super_block cleanup from vfs.  This doesn't lead to any functional
  difference and it's cleaner to do the clean up explicitly for all
  files.

Combined with the previous changes, this makes all cgroup file
creation errors handled correctly.

v2: Added comment on init_cred.

v3: Li spotted that cgroup_mount() wasn't freeing tmp_links after base
    file addition failure.  Fix it by adding free_tmp_links error
    handling label.

v4: v3 introduced build bugs which got noticed by Fengguang's awesome
    kbuild test robot.  Fixed, and shame on me.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>