dm zoned: add metadata pointer to struct dmz_dev

Add a metadata pointer within struct dmz_dev and use it as argument
for blkdev_report_zones() instead of the metadata itself.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>