dm zoned: Introduce dmz_dev_is_dying() and dmz_check_dev()

Introduce accessors dmz_dev_is_dying() and dmz_check_dev() to
avoid having to reference the devices directly.

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