nvdimm/pmem: move dax_attribute_group from dax to pmem

nvdimm/pmem: move dax_attribute_group from dax to pmem

dax_attribute_group is only used by the pmem driver, and can avoid the
completely pointless lookup by the disk name if moved there.  This
leaves just a single caller of dax_get_by_host, so move dax_get_by_host
into the same ifdef block as that caller.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/20210922173431.2454024-3-hch@lst.de
Signed-off-by: Dan Williams <dan.j.williams@intel.com>