While making the root blkg unconditional, ec13b1d6

 ("blkcg: always
create the blkcg_gq for the root blkcg") removed the part which clears
q->root_blkg and ->root_rl.blkg during q exit.  This leaves the two
pointers dangling after blkg_destroy_all().  blk-throttle exit path
performs blkg traversals and dereferences ->root_blkg and can lead to
the following oops.

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000558
 IP: [<ffffffff81389746>] __blkg_lookup+0x26/0x70
 ...
 task: ffff88001b4e2580 ti: ffff88001ac0c000 task.ti: ffff88001ac0c000
 RIP: 0010:[<ffffffff81389746>]  [<ffffffff81389746>] __blkg_lookup+0x26/0x70
 ...
 Call Trace:
  [<ffffffff8138d14a>] blk_throtl_drain+0x5a/0x110
  [<ffffffff8138a108>] blkcg_drain_queue+0x18/0x20
  [<ffffffff81369a70>] __blk_drain_queue+0xc0/0x170
  [<ffffffff8136a101>] blk_queue_bypass_start+0x61/0x80
  [<ffffffff81388c59>] blkcg_deactivate_policy+0x39/0x100
  [<ffffffff8138d328>] blk_throtl_exit+0x38/0x50
  [<ffffffff8138a14e>] blkcg_exit_queue+0x3e/0x50
  [<ffffffff8137016e>] blk_release_queue+0x1e/0xc0
 ...

While the bug is a straigh-forward use-after-free bug, it is tricky to
reproduce because blkg release is RCU protected and the rest of exit
path usually finishes before RCU grace period.

This patch fixes the bug by updating blkg_destro_all() to clear
q->root_blkg and ->root_rl.blkg.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: "Richard W.M. Jones" <rjones@redhat.com>
Reported-by: Josh Boyer <jwboyer@fedoraproject.org>
Link: http://lkml.kernel.org/g/CA+5PVA5rzQ0s4723n5rHBcxQa9t0cW8BPPBekr_9aMRoWt2aYg@mail.gmail.com
Fixes: ec13b1d6

 ("blkcg: always create the blkcg_gq for the root blkcg")
Cc: stable@vger.kernel.org # v4.2+
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>