xfs: factor out a xfs_buftarg_is_dax helper

Refactor the DAX setup code in preparation of removing
bdev_dax_supported.

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/20210826135510.6293-9-hch@lst.de


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