block: cleanup bio_issue

Now that bio->bi_issue is only used by blk-iolatency to get bio issue
time, replace bio_issue with u64 time directly and remove bio_issue to
make code cleaner.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>