aoe: properly initialise the request_queue's backing_dev_info

AOE forgot to initialise its queue's backing_dev_info, so kernels crash.
(http://bugzilla.kernel.org/show_bug.cgi?id=9482)

Fix that and consoldate aoeblk_gdalloc()'s error handling.

Thanks be to Jon for reporting and testing.

Cc: "Ed L. Cashin" <ecashin@coraid.com>
Cc: <stable@kernel.org>
Cc: "Jon Nelson" <jnelson@jamponi.net>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/aoe/aoeblk.c

@@ -6,6 +6,7 @@
 
 #include <linux/hdreg.h>
 #include <linux/blkdev.h>
+#include <linux/backing-dev.h>
 #include <linux/fs.h>
 #include <linux/ioctl.h>
 #include <linux/genhd.h>
@@ -210,25 +211,20 @@ aoeblk_gdalloc(void *vp)
 	if (gd == NULL) {
 		printk(KERN_ERR "aoe: cannot allocate disk structure for %ld.%ld\n",
 			d->aoemajor, d->aoeminor);
-		spin_lock_irqsave(&d->lock, flags);
-		d->flags &= ~DEVFL_GDALLOC;
-		spin_unlock_irqrestore(&d->lock, flags);
-		return;
+		goto err;
 	}
 
 	d->bufpool = mempool_create_slab_pool(MIN_BUFS, buf_pool_cache);
 	if (d->bufpool == NULL) {
 		printk(KERN_ERR "aoe: cannot allocate bufpool for %ld.%ld\n",
 			d->aoemajor, d->aoeminor);
-		put_disk(gd);
-		spin_lock_irqsave(&d->lock, flags);
-		d->flags &= ~DEVFL_GDALLOC;
-		spin_unlock_irqrestore(&d->lock, flags);
-		return;
+		goto err_disk;
 	}
 
-	spin_lock_irqsave(&d->lock, flags);
 	blk_queue_make_request(&d->blkq, aoeblk_make_request);
+	if (bdi_init(&d->blkq.backing_dev_info))
+		goto err_mempool;
+	spin_lock_irqsave(&d->lock, flags);
 	gd->major = AOE_MAJOR;
 	gd->first_minor = d->sysminor * AOE_PARTITIONS;
 	gd->fops = &aoe_bdops;
@@ -246,6 +242,16 @@ aoeblk_gdalloc(void *vp)
 
 	add_disk(gd);
 	aoedisk_add_sysfs(d);
+	return;
+
+err_mempool:
+	mempool_destroy(d->bufpool);
+err_disk:
+	put_disk(gd);
+err:
+	spin_lock_irqsave(&d->lock, flags);
+	d->flags &= ~DEVFL_GDALLOC;
+	spin_unlock_irqrestore(&d->lock, flags);
 }
 
 void