block: Set ELEVATOR_F_ZBD_SEQ_WRITE for nullblk zoned disks

Using the helper blk_queue_required_elevator_features(), set the
elevator feature ELEVATOR_F_ZBD_SEQ_WRITE as required for the request
queue of null_blk devices created with zoned mode enabled.

This feature requirement can always be satisfied as the mq-deadline
elevator is always selected for in-kernel compilation when
CONFIG_BLK_DEV_ZONED (zoned block device support) is enabled.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk_main.c

@@ -1695,6 +1695,8 @@ static int null_add_dev(struct nullb_device *dev)
 		blk_queue_chunk_sectors(nullb->q, dev->zone_size_sects);
 		nullb->q->limits.zoned = BLK_ZONED_HM;
 		blk_queue_flag_set(QUEUE_FLAG_ZONE_RESETALL, nullb->q);
+		blk_queue_required_elevator_features(nullb->q,
+						ELEVATOR_F_ZBD_SEQ_WRITE);
 	}
 
 	nullb->q->queuedata = nullb;