This patch adds cfent (cgroup file entry) which is the association
between a cgroup and a file.  This is in-cgroup representation of
files under a cgroup directory.  This simplifies walking walking
cgroup files and thus cgroup_clear_directory(), which is now
implemented in two parts - cgroup_rm_file() and a loop around it.

cgroup_rm_file() will be used to implement cftype removal and cfent is
scheduled to serve cgroup specific per-file data (e.g. for sysfs-like
"sever" semantics).

v2: - cfe was freed from cgroup_rm_file() which led to use-after-free
      if the file had openers at the time of removal.  Moved to
      cgroup_diput().

    - cgroup_clear_directory() triggered WARN_ON_ONCE() if d_subdirs
      wasn't empty after removing all files.  This triggered
      spuriously if some files were open during directory clearing.
      Removed.

v3: - In cgroup_diput(), WARN_ONCE(!list_empty(&cfe->node)) could be
      spuriously triggered for root cgroups because they don't go
      through cgroup_clear_directory() on unmount.  Don't trigger WARN
      for root cgroups.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizf@cn.fujitsu.com>
Cc: Glauber Costa <glommer@parallels.com>