test_overflow: fix an IS_ERR() vs NULL bug

root_device_register() returns error pointers, it never returns NULL.

Fixes: ca90800a91ba ("test_overflow: Add memory allocation overflow tests")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

lib/test_overflow.c

@@ -367,7 +367,7 @@ static int __init test_overflow_allocation(void)
 
 	/* Create dummy device for devm_kmalloc()-family tests. */
 	dev = root_device_register(device_name);
-	if (!dev) {
+	if (IS_ERR(dev)) {
 		pr_warn("Cannot register test device\n");
 		return 1;
 	}