While refactoring cgroup file removal path, 05ef1d7c "cgroup:
introduce struct cfent" incorrectly changed the @dir argument of
simple_unlink() to the inode of the file being deleted instead of that
of the containing directory.

The effect of this bug is minor - ctime and mtime of the parent
weren't properly updated on file deletion.

Fix it by using @cgrp->dentry->d_inode instead.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NAl Viro <viro@ZenIV.linux.org.uk>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: stable@vger.kernel.org