xfs: factor out a xfs_setup_dax_always helper

Factor out another DAX setup helper to simplify future changes.  Also
move the experimental warning after the checks to not clutter the log
too much if the setup failed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20211129102203.2243509-8-hch@lst.de


Signed-off-by: Dan Williams <dan.j.williams@intel.com>