drbd: set ->bi_bdev in drbd_req_new

Make sure the newly allocated bio has the correct bi_bdev set from the
start.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Link: https://lore.kernel.org/r/20220202160109.108149-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>