cgroups: make cgroup config a submenu

Making CGROUP related configs be a sub-menu.

This patch make CGROUP related configs be a sub-menu and makes 1st level
configs of "General Setup" shorter.

 including following additional changes
  - add help comment about CGROUPS and GROUP_SCHED.
  - moved MM_OWNER config to the bottom.
    (for good indent in menuconfig)

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>