block: store a block_device pointer in struct bio

Replace the gendisk pointer in struct bio with a pointer to the newly
improved struct block device.  From that the gendisk can be trivially
accessed with an extra indirection, but it also allows to directly
look up all information related to partition remapping.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fs/direct-io.c b/fs/direct-io.c
index d53fa92..2660e74 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -434,7 +434,7 @@ static inline void dio_bio_submit(struct dio *dio, struct dio_submit *sdio)
 	if (dio->is_async && dio->op == REQ_OP_READ && dio->should_dirty)
 		bio_set_pages_dirty(bio);
 
-	dio->bio_disk = bio->bi_disk;
+	dio->bio_disk = bio->bi_bdev->bd_disk;
 
 	if (sdio->submit_io) {
 		sdio->submit_io(bio, dio->inode, sdio->logical_offset_in_bio);