fix: 修复 #1387 错误的把sys_exit进行更改的bug (#1388)

Signed-off-by: longjin <longjin@DragonOS.org>

kernel/src/process/syscall/sys_exit.rs

@@ -20,7 +20,9 @@ impl Syscall for SysExit {
 
     fn handle(&self, args: &[usize], _frame: &mut TrapFrame) -> Result<usize, SystemError> {
         let exit_code = Self::exit_code(args);
-        ProcessManager::group_exit((exit_code & 0xff) << 8);
+        // 仿照 Linux sys_exit：只取低 8 位并左移 8 位，形成 wstatus 编码，
+        // 然后只退出当前线程（不退出整个线程组）。
+        ProcessManager::exit((exit_code & 0xff) << 8);
     }
 
     fn entry_format(&self, args: &[usize]) -> Vec<FormattedSyscallParam> {