xfs: drop experimental warning for FSDAX

FSDAX and reflink can work together now, let's drop this warning.

Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>