CFQ is broken for CONFIG_BLK_CGROUP=y, CFQ_GROUP_IOSCHED=n

[Dmitry Monakhov <dmonakhov@openvz.org>]

I've had an  oops on kernel boot due to NULL pointer deference
linux-2.6-block/for-next HEAD:7eaed1226ab411ee5dc8c34fc0d8034e4c98e3c6
I've enabled CONFIG_BLK_CGROUP, but not CFQ_GROUP_IOSCHED
In this case cfq_ref_get_cfqg() defined as 
static inline struct cfq_group *cfq_ref_get_cfqg(struct cfq_group *cfqg)
{
        return NULL;
}
So following call trace is simply NOOP
cfq_set_request()
 rq->elevator_private3 = cfq_ref_get_cfqg(cfqq->cfqg);

Which later result in OOPS on bio insertion
cfq_insert_request
 -> blkiocg_update_io_add_stats(&(RQ_CFQG(rq))->blkg,...)
    -> spin_lock_irqsave(&blkg->stats_lock, flags);

Bad commit.
>From 7f1dc8a2d2f45fc557b27fd56115338b1d34fc24 Mon Sep 17 00:00:00 2001
From: Vivek Goyal <vgoyal@redhat.com>
Date: Wed, 21 Apr 2010 17:44:16 +0200
Subject: [PATCH] blkio: Fix blkio crash during rq stat update