memcg: memory cgroup hierarchy feature selector

memcg: memory cgroup hierarchy feature selector

Don't enable multiple hierarchy support by default.  This patch introduces
a features element that can be set to enable the nested depth hierarchy
feature.  This feature can only be enabled when the cgroup for which the
feature this is enabled, has no children.

Signed-off-by: Balbir Singh <[email protected]>
Cc: YAMAMOTO Takashi <[email protected]>
Cc: Paul Menage <[email protected]>
Cc: Li Zefan <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Pavel Emelianov <[email protected]>
Cc: Dhaval Giani <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>