cgroup: clean up cgroup.h

- replace old name 'cont' with 'cgrp' (Paul Menage did this cleanup for
  cgroup.c in commit bd89aabc

)
- remove a duplicate declaration of cgroup_path()

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>