dm zoned: fix uninitialized pointer dereference

Make sure that the local variable rzone in dmz_do_reclaim() is always
initialized before being used for printing debug messages.

Fixes: f97809ae

 ("dm zoned: per-device reclaim")
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>