dm zoned: replace 'target' pointer in the bio context

Replace the 'target' pointer in the bio context with the
device pointer as this is what's actually used.

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>