dm: remove useless md->nr_zones variable

md->nr_zones is no longer used for anything. Remove it.

Signed-off-by: Benjamin Marzinski <[e-mail]>
Reviewed-by: Damien Le Moal <[e-mail]>
Signed-off-by: Mikulas Patocka <[e-mail]>

drivers/md/dm-core.h

···
 	struct srcu_struct io_barrier;
 
 #ifdef CONFIG_BLK_DEV_ZONED
-	unsigned int nr_zones;
 	void *zone_revalidate_map;
 	struct task_struct *revalidate_map_task;
 #endif

drivers/md/dm-zone.c

···
 		return ret;
 	}
 
-	md->nr_zones = disk->nr_zones;
-
 	return 0;
 }
 
···
 			set_bit(DMF_EMULATE_ZONE_APPEND, &md->flags);
 	} else {
 		clear_bit(DMF_EMULATE_ZONE_APPEND, &md->flags);
-		md->nr_zones = 0;
 		md->disk->nr_zones = 0;
 	}
 }