block: set up rq->rq_disk properly for flush requests

q->bar_rq.rq_disk is NULL. Use the rq_disk of the original request
instead.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>