dm zoned: avoid unnecessary device recalulation for secondary superblock

The secondary superblock must reside on the same device as the primary
superblock, so there is no need to re-calculate the device.

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>