block: reorder struct bio to remove padding on 64bit

Remove 8 bytes of padding from struct bio which also removes 16 bytes from
struct bio_pair to make it 248 bytes.  bio_pair then fits into one fewer
cache lines & into a smaller slab.
Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

include/linux/bio.h

@@ -90,10 +90,11 @@ struct bio {
 
 	unsigned int		bi_comp_cpu;	/* completion CPU */
 
+	atomic_t		bi_cnt;		/* pin count */
+
 	struct bio_vec		*bi_io_vec;	/* the actual vec list */
 
 	bio_end_io_t		*bi_end_io;
-	atomic_t		bi_cnt;		/* pin count */
 
 	void			*bi_private;
 #if defined(CONFIG_BLK_DEV_INTEGRITY)