nvme-pci: skip nvme_write_sq_db on empty rqlist

nvme_submit_cmds() should check the rqlist before calling
nvme_write_sq_db(); if the list is empty, it must return immediately.

Fixes: beadf00 ("nvme-pci: reverse request order in nvme_queue_rqs")
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>

drivers/nvme/host/pci.c

@@ -986,6 +986,9 @@ static void nvme_submit_cmds(struct nvme_queue *nvmeq, struct rq_list *rqlist)
 {
 	struct request *req;
 
+	if (rq_list_empty(rqlist))
+		return;
+
 	spin_lock(&nvmeq->sq_lock);
 	while ((req = rq_list_pop(rqlist))) {
 		struct nvme_iod *iod = blk_mq_rq_to_pdu(req);