xfs: disable xfs_ioc_space for always COW inodes

If we always have to write out of place preallocating blocks is
pointless.  We already check for this in the normal falloc path, but
the check was missig in the legacy ALLOCSP path.

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>