dm zoned: fix potential NULL dereference in dmz_do_reclaim()

This function is supposed to return error pointers so it matches the
dmz_get_rnd_zone_for_reclaim() function.  The current code could lead to
a NULL dereference in dmz_do_reclaim()

Fixes: b234c6d7 ("dm zoned: improve error handling in reclaim")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-zoned-metadata.c

@@ -1588,7 +1588,7 @@ static struct dm_zone *dmz_get_seq_zone_for_reclaim(struct dmz_metadata *zmd)
 	struct dm_zone *zone;
 
 	if (list_empty(&zmd->map_seq_list))
-		return NULL;
+		return ERR_PTR(-EBUSY);
 
 	list_for_each_entry(zone, &zmd->map_seq_list, link) {
 		if (!zone->bzone)
@@ -1597,7 +1597,7 @@ static struct dm_zone *dmz_get_seq_zone_for_reclaim(struct dmz_metadata *zmd)
 			return zone;
 	}
 
-	return NULL;
+	return ERR_PTR(-EBUSY);
 }
 
 /*