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 <[e-mail]>
Acked-by: Daisuke Nishimura <[e-mail]>
Reviewed-by: Johannes Weiner <[e-mail]>
Cc: Balbir Singh <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memcontrol.c

···
 	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.