Btrfs: fix NULL pointer crash of deleting a seed device

Same as normal devices, seed devices should be initialized with
fs_info->dev_root as well, otherwise we'll get a NULL pointer crash.

Cc: Chris Murphy <[e-mail]>
Reported-by: Chris Murphy <[e-mail]>
Signed-off-by: Liu Bo <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/volumes.c

···
 	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
 	struct btrfs_device *device;
 
-	mutex_lock(&fs_devices->device_list_mutex);
-	list_for_each_entry(device, &fs_devices->devices, dev_list)
-		device->dev_root = fs_info->dev_root;
-	mutex_unlock(&fs_devices->device_list_mutex);
+	while (fs_devices) {
+		mutex_lock(&fs_devices->device_list_mutex);
+		list_for_each_entry(device, &fs_devices->devices, dev_list)
+			device->dev_root = fs_info->dev_root;
+		mutex_unlock(&fs_devices->device_list_mutex);
+
+		fs_devices = fs_devices->seed;
+	}
 }
 
 static void __btrfs_reset_dev_stats(struct btrfs_device *dev)