NVMe: Reduce maximum queue depth by 1

The spec says we're not allowed to completely fill the submission queue.
Solve this by reducing the number of allocatable cmdids by 1.
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>

drivers/block/nvme.c

@@ -156,7 +156,7 @@ static struct nvme_cmd_info *nvme_cmd_info(struct nvme_queue *nvmeq)
 static int alloc_cmdid(struct nvme_queue *nvmeq, void *ctx, int handler,
 							unsigned timeout)
 {
-	int depth = nvmeq->q_depth;
+	int depth = nvmeq->q_depth - 1;
 	struct nvme_cmd_info *info = nvme_cmd_info(nvmeq);
 	int cmdid;