device-dax: fix 'passing zero to ERR_PTR()' warning

Dan Carpenter reports:

    The patch 7b6be8444e0f: "dax: refactor dax-fs into a generic provider
    of 'struct dax_device' instances" from Apr 11, 2017, leads to the
    following static checker warning:

        drivers/dax/device.c:643 devm_create_dev_dax()
        warn: passing zero to 'ERR_PTR'

Fix the case where we inadvertently leak 0 to ERR_PTR() by setting at
every error case, and make it clear that 'count' is never 0.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dax/device.c

@@ -567,7 +567,10 @@ struct dev_dax *devm_create_dev_dax(struct dax_region *dax_region,
 	struct inode *inode;
 	struct device *dev;
 	struct cdev *cdev;
-	int rc = 0, i;
+	int rc, i;
+
+	if (!count)
+		return ERR_PTR(-EINVAL);
 
 	dev_dax = kzalloc(sizeof(*dev_dax) + sizeof(*res) * count, GFP_KERNEL);
 	if (!dev_dax)
@@ -598,8 +601,10 @@ struct dev_dax *devm_create_dev_dax(struct dax_region *dax_region,
 	 * device outside of mmap of the resulting character device.
 	 */
 	dax_dev = alloc_dax(dev_dax, NULL, NULL);
-	if (!dax_dev)
+	if (!dax_dev) {
+		rc = -ENOMEM;
 		goto err_dax;
+	}
 
 	/* from here on we're committed to teardown via dax_dev_release() */
 	dev = &dev_dax->dev;