Update thread and syscall to use local fd counter instead global one, preventing overflow

src/userprog/syscall.c

@@ -16,8 +16,6 @@
 #define MAX_SYSCALL_ARGS 3
 #define EXIT_FAILURE -1
 
-static unsigned fd_counter = MIN_USER_FD;
-
 struct open_file
   {
     int fd;                     /* File Descriptor / Identifier */
@@ -224,7 +222,7 @@ syscall_open (const char *file)
     }
 
   /* Populate the above struct, with a unique FD and the current open file */
-  file_info->fd = fd_counter++;
+  file_info->fd = thread_current ()->fd_counter++;
   file_info->file = ptr;
 
   /* Add the new FD->file mapping to the hashtable for the current thread */