cgroup: switch to proc_create()

There is a race between create_proc_entry() and the assignment of file ops.
proc_create() is invented to fix it.

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>