Btrfs: use mnt_want_write_file instead of mnt_want_write

mnt_want_write_file is faster when file has been opened for write.
Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>

fs/btrfs/ioctl.c

@@ -3265,7 +3265,7 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
-	ret = mnt_want_write(file->f_path.mnt);
+	ret = mnt_want_write_file(file);
 	if (ret)
 		return ret;
 
@@ -3335,7 +3335,7 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
 out:
 	mutex_unlock(&fs_info->balance_mutex);
 	mutex_unlock(&fs_info->volume_mutex);
-	mnt_drop_write(file->f_path.mnt);
+	mnt_drop_write_file(file);
 	return ret;
 }