block: remove 16 bytes of padding from struct request on 64bits

Remove alignment padding to shrink struct request from 336 to 320 bytes
so needing one fewer cacheline and therefore removing 48 bytes from
struct request_queue.

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>