sd-device: remove holes in struct sd_device

sd-device: remove holes in struct sd_device

Normally, we don't care too much about what pahole reports. But this structure
could potentially be allocated for every device on the system, i.e. in a large
number of copies. 5 vs 7 cache lines is nice.

/* size: 400, cachelines: 7, members: 53 */
/* sum members: 330, holes: 12, sum holes: 70 */
/* last cacheline: 16 bytes */

/* size: 320, cachelines: 5, members: 53 */
/* bit holes: 1, sum bit holes: 6 bits */
/* bit_padding: 5 bits */