xfs: ignore extent size hints for always COW inodes

There is no point in applying extent size hints for always COW inodes,
as we would just have to COW any extra allocation beyond the data
actually written.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>