xfs: remove experimental tag for reflinks

But reject reflink + DAX file systems for now until the code to
support reflinks on DAX is actually implemented.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: port to 4.16]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>