memcg: bugfix check mem_cgroup_disabled() at split fixup

mem_cgroup_disabled() should be checked at splitting.  If disabled, no
heavy work is necesary.
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -2145,6 +2145,8 @@ void mem_cgroup_split_huge_fixup(struct page *head, struct page *tail)
 	struct page_cgroup *tail_pc = lookup_page_cgroup(tail);
 	unsigned long flags;
 
+	if (mem_cgroup_disabled())
+		return;
 	/*
 	 * We have no races with charge/uncharge but will have races with
 	 * page state accounting.