nvme: strip trailing 0-bytes in wwid_show

Some broken controllers (such as earlier Linux targets) pad model or
serial fields with 0-bytes rather than spaces. The NVMe spec disallows
0 bytes in "ASCII" fields.  Thus strip trailing 0-bytes, too. Also make
sure that we get no underflow for pathological input.

Signed-off-by: Martin Wilck <mwilck@suse.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/core.c

@@ -2004,9 +2004,11 @@ static ssize_t wwid_show(struct device *dev, struct device_attribute *attr,
 	if (memchr_inv(ns->eui, 0, sizeof(ns->eui)))
 		return sprintf(buf, "eui.%8phN\n", ns->eui);
 
-	while (ctrl->serial[serial_len - 1] == ' ')
+	while (serial_len > 0 && (ctrl->serial[serial_len - 1] == ' ' ||
+				  ctrl->serial[serial_len - 1] == '\0'))
 		serial_len--;
-	while (ctrl->model[model_len - 1] == ' ')
+	while (model_len > 0 && (ctrl->model[model_len - 1] == ' ' ||
+				 ctrl->model[model_len - 1] == '\0'))
 		model_len--;
 
 	return sprintf(buf, "nvme.%04x-%*phN-%*phN-%08x\n", ctrl->vid,