Refactor validate_user_string to remove unnecessary variable to track length of str

src/userprog/syscall.c

@@ -446,13 +446,12 @@ validate_user_string (const char *str)
   if (str == NULL || !is_user_vaddr (str))
     syscall_exit (EXIT_FAILURE);
 
-  size_t length = 0;
   size_t offset = (uintptr_t) str % PGSIZE;
 
   /* We move page by page, checking if the page is mapped to physical memory. */
   for (;;)
   {
-    void *page = pg_round_down (str + length);
+    void *page = pg_round_down (str);
 
     if (!is_user_vaddr(page) ||
         pagedir_get_page (thread_current ()->pagedir, page) == NULL)