[PATCH] xfs: fix extent format buffer allocation size

[Christoph Hellwig]

> +	xfs_bmbt_rec_t		*ext_buffer;
> +
> +	ext_buffer = kmem_alloc(XFS_IFORK_SIZE(ip, whichfork),

If the fork size be the minimum of XFS_IFORK_SIZE and the if_bytes
value?