block: needs to set the residual length of a bidi request

Tejun's "block: set rq->resid_len to blk_rq_bytes() on issue" patch
seems to be incomplete; It doesn't set rq->resid_len to blk_rq_bytes()
for a bidi request (req->next_rq). As a result, all bidi users are
broken.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>