memcg: avoid unnecessary initialization

* remove over-killing initialization (in fast path)
* makeing the condition for PAGE_CGROUP_FLAG_ACTIVE be more obvious.

Signed-off-by: KAMEAZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>