nvme: print a warning for when listing active namespaces fails

During the scan_work, an Identify command is issued to figure out which
namespaces are active.  If this command fails, the nvme driver falls back
to scanning namespaces sequentially.  In this situation, we don't see
any warnings and don't even know whether list-ns command has been failed
or not easiliy.

Printa warning when the Identify command executin fail:

[    1.108399] nvme nvme0: Identify NS List failed (status=0x400b)
[    1.109583] nvme0n1: detected capacity change from 0 to 1048576
[    1.112186] nvme nvme0: Identify Descriptors failed (nsid=2, status=0x4002)
[    1.113929] nvme nvme0: Identify Descriptors failed (nsid=3, status=0x4002)
[    1.116537] nvme nvme0: Identify Descriptors failed (nsid=4, status=0x4002)
...
Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/core.c

@@ -4110,8 +4110,11 @@ static int nvme_scan_ns_list(struct nvme_ctrl *ctrl)
 
 		ret = nvme_submit_sync_cmd(ctrl->admin_q, &cmd, ns_list,
 					    NVME_IDENTIFY_DATA_SIZE);
-		if (ret)
+		if (ret) {
+			dev_warn(ctrl->device,
+				"Identify NS List failed (status=0x%x)\n", ret);
 			goto free;
+		}
 
 		for (i = 0; i < nr_entries; i++) {
 			u32 nsid = le32_to_cpu(ns_list[i]);