block: reorder bio::__bi_remaining for better packing

Simple reordering of __bi_remaining can reduce bio size by 8 bytes that
are now wasted on padding (measured on x86_64):

struct bio {
        struct bio *               bi_next;              /*     0     8 */
        struct gendisk *           bi_disk;              /*     8     8 */
        unsigned int               bi_opf;               /*    16     4 */
        short unsigned int         bi_flags;             /*    20     2 */
        short unsigned int         bi_ioprio;            /*    22     2 */
        short unsigned int         bi_write_hint;        /*    24     2 */
        blk_status_t               bi_status;            /*    26     1 */
        u8                         bi_partno;            /*    27     1 */

        /* XXX 4 bytes hole, try to pack */

        struct bvec_iter   bi_iter;                      /*    32    24 */

        /* XXX last struct has 4 bytes of padding */

        atomic_t                   __bi_remaining;       /*    56     4 */

        /* XXX 4 bytes hole, try to pack */
[...]
        /* size: 104, cachelines: 2, members: 19 */
        /* sum members: 96, holes: 2, sum holes: 8 */
        /* paddings: 1, sum paddings: 4 */
        /* last cacheline: 40 bytes */
};

Now becomes:

struct bio {
        struct bio *               bi_next;              /*     0     8 */
        struct gendisk *           bi_disk;              /*     8     8 */
        unsigned int               bi_opf;               /*    16     4 */
        short unsigned int         bi_flags;             /*    20     2 */
        short unsigned int         bi_ioprio;            /*    22     2 */
        short unsigned int         bi_write_hint;        /*    24     2 */
        blk_status_t               bi_status;            /*    26     1 */
        u8                         bi_partno;            /*    27     1 */
        atomic_t                   __bi_remaining;       /*    28     4 */
        struct bvec_iter   bi_iter;                      /*    32    24 */

        /* XXX last struct has 4 bytes of padding */
[...]
        /* size: 96, cachelines: 2, members: 19 */
        /* paddings: 1, sum paddings: 4 */
        /* last cacheline: 32 bytes */
};
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blk_types.h

@@ -153,10 +153,10 @@ struct bio {
 	unsigned short		bi_write_hint;
 	blk_status_t		bi_status;
 	u8			bi_partno;
+	atomic_t		__bi_remaining;
 
 	struct bvec_iter	bi_iter;
 
-	atomic_t		__bi_remaining;
 	bio_end_io_t		*bi_end_io;
 
 	void			*bi_private;