block: replace bi_bdev with a gendisk pointer and partitions index

This way we don't need a block_device structure to submit I/O.  The
block_device has different life time rules from the gendisk and
request_queue and is usually only available when the block device node
is open.  Other callers need to explicitly create one (e.g. the lightnvm
passthrough code, or the new nvme multipathing code).

For the actual I/O path all that we need is the gendisk, which exists
once per block device.  But given that the block layer also does
partition remapping we additionally need a partition index, which is
used for said remapping in generic_make_request.

Note that all the block drivers generally want request_queue or
sometimes the gendisk, so this removes a layer of indirection all
over the stack.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/dm-zoned-target.c

@@ -238,7 +238,7 @@ static void dmz_submit_write_bio(struct dmz_target *dmz, struct dm_zone *zone,
 	struct dmz_bioctx *bioctx = dm_per_bio_data(bio, sizeof(struct dmz_bioctx));
 
 	/* Setup and submit the BIO */
-	bio->bi_bdev = dmz->dev->bdev;
+	bio_set_dev(bio, dmz->dev->bdev);
 	bio->bi_iter.bi_sector = dmz_start_sect(dmz->metadata, zone) + dmz_blk2sect(chunk_block);
 	atomic_inc(&bioctx->ref);
 	generic_make_request(bio);
@@ -586,7 +586,7 @@ static int dmz_map(struct dm_target *ti, struct bio *bio)
 		      (unsigned long long)dmz_chunk_block(dmz->dev, dmz_bio_block(bio)),
 		      (unsigned int)dmz_bio_blocks(bio));
 
-	bio->bi_bdev = dev->bdev;
+	bio_set_dev(bio, dev->bdev);
 
 	if (!nr_sectors && bio_op(bio) != REQ_OP_WRITE)
 		return DM_MAPIO_REMAPPED;