writeback: check for registered bdi in flusher add and inode dirty

Also a debugging aid. We want to catch dirty inodes being added to
backing devices that don't do writeback.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

mm/backing-dev.c

@@ -465,6 +465,12 @@ void static bdi_add_default_flusher_task(struct backing_dev_info *bdi)
 	if (!bdi_cap_writeback_dirty(bdi))
 		return;
 
+	if (WARN_ON(!test_bit(BDI_registered, &bdi->state))) {
+		printk(KERN_ERR "bdi %p/%s is not registered!\n",
+							bdi, bdi->name);
+		return;
+	}
+
 	/*
 	 * Check with the helper whether to proceed adding a task. Will only
 	 * abort if we two or more simultanous calls to
@@ -528,6 +534,7 @@ int bdi_register(struct backing_dev_info *bdi, struct device *parent,
 	}
 
 	bdi_debug_register(bdi, dev_name(dev));
+	set_bit(BDI_registered, &bdi->state);
 exit:
 	return ret;
 }