xfs: move dax device handling into xfs_{alloc,free}_buftarg

Hide the DAX device lookup from the xfs_super.c code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/20211129102203.2243509-22-hch@lst.de


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