Btrfs: fix unlock order in btrfs_ioctl_rm_dev

Fix unlock order in btrfs_ioctl_rm_dev().
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

fs/btrfs/ioctl.c

@@ -2319,8 +2319,8 @@ static long btrfs_ioctl_rm_dev(struct file *file, void __user *arg)
 	kfree(vol_args);
 out:
 	mutex_unlock(&root->fs_info->volume_mutex);
-	mnt_drop_write_file(file);
 	atomic_set(&root->fs_info->mutually_exclusive_operation_running, 0);
+	mnt_drop_write_file(file);
 	return ret;
 }