nvmet: avoid queuing keep-alive timer if it is disabled

Issue following command:
nvme set-feature -f 0xf -v 0 /dev/nvme1n1 # disable keep-alive timer
nvme admin-passthru -o 0x18 /dev/nvme1n1  # send keep-alive command
will make keep-alive timer fired and thus delete the controller like
below:

[247459.907635] nvmet: ctrl 1 keep-alive timer (0 seconds) expired!
[247459.930294] nvmet: ctrl 1 fatal error occurred!

Avoid this by not queuing delayed keep-alive if it is disabled when
keep-alive command is received from the admin queue.
Signed-off-by: Hou Pu <houpu.main@gmail.com>
Tested-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/target/admin-cmd.c

@@ -919,15 +919,21 @@ void nvmet_execute_async_event(struct nvmet_req *req)
 void nvmet_execute_keep_alive(struct nvmet_req *req)
 {
 	struct nvmet_ctrl *ctrl = req->sq->ctrl;
+	u16 status = 0;
 
 	if (!nvmet_check_transfer_len(req, 0))
 		return;
 
+	if (!ctrl->kato) {
+		status = NVME_SC_KA_TIMEOUT_INVALID;
+		goto out;
+	}
+
 	pr_debug("ctrl %d update keep-alive timer for %d secs\n",
 		ctrl->cntlid, ctrl->kato);
-
 	mod_delayed_work(system_wq, &ctrl->ka_work, ctrl->kato * HZ);
-	nvmet_req_complete(req, 0);
+out:
+	nvmet_req_complete(req, status);
 }
 
 u16 nvmet_parse_admin_cmd(struct nvmet_req *req)