block: align blkdev_dio inlined bio to a cacheline

We get all sorts of unreliable and funky results since the bio is
designed to align on a cacheline, which it does not when inlined like
this.

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

block/fops.c

@@ -137,7 +137,7 @@ struct blkdev_dio {
 	size_t			size;
 	atomic_t		ref;
 	unsigned int		flags;
-	struct bio		bio;
+	struct bio		bio ____cacheline_aligned_in_smp;
 };
 
 static struct bio_set blkdev_dio_pool;