ublk: fix 'warn: variable dereferenced before check 'req'' from Smatch

The added check of 'req_op(req) == REQ_OP_ZONE_APPEND' should have been
done after the request is confirmed as valid.

Actually here, the request should always been true, so add one
WARN_ON_ONCE(!req), meantime move the zone_append check after
checking the request.

Cc: Andreas Hindborg <a.hindborg@samsung.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Fixes: 29802d7c ("ublk: enable zoned storage support")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230811135216.420404-1-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -1397,11 +1397,13 @@ static void ublk_commit_completion(struct ublk_device *ub,
 
 	/* find the io request and complete */
 	req = blk_mq_tag_to_rq(ub->tag_set.tags[qid], tag);
+	if (WARN_ON_ONCE(unlikely(!req)))
+		return;
 
 	if (req_op(req) == REQ_OP_ZONE_APPEND)
 		req->__sector = ub_cmd->zone_append_lba;
 
-	if (req && likely(!blk_should_fake_timeout(req->q)))
+	if (likely(!blk_should_fake_timeout(req->q)))
 		ublk_put_req_ref(ubq, req);
 }