[RFC] splitting cgroup.c

While working on cgroup xattr, it appeared to me it's better to
create a cgroup_xattr.c instead of stuffing things into cgroup.c.

Then I took a look at how big it is.

    $ ls -l -S kernel/*.c
    -rw-rw-r--  1 lizf lizf 142717 Jan 18 10:15 cgroup.c
    -rw-rw-r--  1 lizf lizf 106498 Jan 18 10:15 workqueue.c
    -rw-rw-r--  1 lizf lizf 105206 Jan 18 10:15 lockdep.c
    -rw-rw-r--  1 lizf lizf  91321 Jan 18 10:16 module.c
    
    $ wc -l kernel/*.c | sort -n -r
      90397 total
       5289 cgroup.c
       4206 lockdep.c
       3840 workqueue.c
       3507 module.c

So I think for the sake of readability and maintainability, we'd
better split cgroup.c into smaller pieces:

- event_control.c
- css_id.c
- debug.c
- proc.c
- super.c
...
- cgroup.c

We may add xattr.c and release_agency.c as I'm working on them.

Any objection?