cfq-iosched: Fixed boot warning with BLK_CGROUP=y and CFQ_GROUP_IOSCHED=n

cfq-iosched: Fixed boot warning with BLK_CGROUP=y and CFQ_GROUP_IOSCHED=n

Hi Jens,

Few days back Ingo noticed a CFQ boot time warning. This patch fixes it.
The issue here is that with CFQ_GROUP_IOSCHED=n, CFQ should not really
be making blkio stat related calls.

> Hm, it's still not entirely fixed, as of 2.6.35-rc2-00131-g7908a9e. With
> some
> configs i get bad spinlock warnings during bootup:
>
> [   28.968013] initcall net_olddevs_init+0x0/0x82 returned 0 after 93750
> usecs
> [   28.972003] calling  b44_init+0x0/0x55 @ 1
> [   28.976009] bus: 'pci': add driver b44
> [   28.976374]  sda:
> [   28.978157] BUG: spinlock bad magic on CPU#1, async/0/117
> [   28.980000]  lock: 7e1c5bbc, .magic: 00000000, .owner: <none>/-1, +.owner_cpu: 0
> [   28.980000] Pid: 117, comm: async/0 Not tainted +2.6.35-rc2-tip-01092-g010e7ef-dirty #8183
> [   28.980000] Call Trace:
> [   28.980000]  [<41ba6d55>] ? printk+0x20/0x24
> [   28.980000]  [<4134b7b7>] spin_bug+0x7c/0x87
> [   28.980000]  [<4134b853>] do_raw_spin_lock+0x1e/0x123
> [   28.980000]  [<41ba92ca>] ? _raw_spin_lock_irqsave+0x12/0x20
> [   28.980000]  [<41ba92d2>] _raw_spin_lock_irqsave+0x1a/0x20
> [   28.980000]  [<4133476f>] blkiocg_update_io_add_stats+0x25/0xfb
> [   28.980000]  [<41335dae>] ? cfq_prio_tree_add+0xb1/0xc1
> [   28.980000]  [<41337bc7>] cfq_insert_request+0x8c/0x425

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>