fix bug in do_fork function

labcodes_answer/lab8_result/kern/process/proc.c

@@ -484,7 +484,7 @@ do_fork(uint32_t clone_flags, uintptr_t stack, struct trapframe *tf) {
         goto bad_fork_cleanup_kstack;
     }
     if (copy_mm(clone_flags, proc) != 0) {
-        goto bad_fork_cleanup_kstack;
+        goto bad_fork_cleanup_fs;
     }
     copy_thread(proc, stack, tf);