block: init flush rq ref count to 1

We discovered a problem in newer kernels where a disconnect of a NBD
device while the flush request was pending would result in a hang.  This
is because the blk mq timeout handler does

        if (!refcount_inc_not_zero(&rq->ref))
                return true;

to determine if it's ok to run the timeout handler for the request.
Flush_rq's don't have a ref count set, so we'd skip running the timeout
handler for this request and it would just sit there in limbo forever.

Fix this by always setting the refcount of any request going through
blk_init_rq() to 1.  I tested this with a nbd-server that dropped flush
requests to verify that it hung, and then tested with this patch to
verify I got the timeout as expected and the error handling kicked in.
Thanks,

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>