dm: use fs_dax_get_by_bdev instead of dax_get_by_host

dm: use fs_dax_get_by_bdev instead of dax_get_by_host

There is no point in trying to finding the dax device if the DAX flag is
not set on the queue as none of the users of the device mapper exported
block devices could make use of the DAX capability.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Link: https://lore.kernel.org/r/20210826135510.6293-4-hch@lst.de
Signed-off-by: Dan Williams <dan.j.williams@intel.com>