lib/string/strchr/: strnul(): Simplify implementation

This avoids an __auto_type local variable, and a GNU statement
expression.

Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

Author: alejandro-colomar

lib/string/strchr/strnul.h

@@ -1,4 +1,4 @@
-// SPDX-FileCopyrightText: 2024, Alejandro Colomar <alx@kernel.org>
+// SPDX-FileCopyrightText: 2024-2026, Alejandro Colomar <alx@kernel.org>
 // SPDX-License-Identifier: BSD-3-Clause
 
 
@@ -10,17 +10,9 @@
 
 #include <string.h>
 
-#include "attr.h"
 
-
-// string null-byte
-// Similar to strlen(3), but return a pointer instead of an offset.
-#define strnul(s)                                                             \
-({                                                                            \
-	__auto_type  s_ = s;                                                  \
-	                                                                      \
-	s_ + strlen(s_);                                                      \
-})
+// strnul - string null-byte
+#define strnul(s)  strchr(s, '\0')
 
 
 #endif  // include guard