nvme-pci: fix out of bounds access in nvme_cqe_pending

There is an out of bounds array access in nvme_cqe_peding().

When enable irq_thread for nvme interrupt, there is racing between the
nvmeq->cq_head updating and reading.

nvmeq->cq_head is updated in nvme_update_cq_head(), if nvmeq->cq_head
equals nvmeq->q_depth and before its value set to zero, nvme_cqe_pending()
uses its value as an array index, the index will be out of bounds.
Signed-off-by: NHongbo Yao <yaohongbo@huawei.com>
[hch: slight coding style update]
Signed-off-by: NChristoph Hellwig <hch@lst.de>

drivers/nvme/host/pci.c

@@ -1020,9 +1020,11 @@ static void nvme_complete_cqes(struct nvme_queue *nvmeq, u16 start, u16 end)
 
 static inline void nvme_update_cq_head(struct nvme_queue *nvmeq)
 {
-	if (++nvmeq->cq_head == nvmeq->q_depth) {
+	if (nvmeq->cq_head == nvmeq->q_depth - 1) {
 		nvmeq->cq_head = 0;
 		nvmeq->cq_phase = !nvmeq->cq_phase;
+	} else {
+		nvmeq->cq_head++;
 	}
 }