Fix the SIGKILL fd leak bug.

src/syscall/process.rs

@@ -43,8 +43,7 @@ pub fn exit(status: usize) -> ! {
             let mut context = context_lock.write();
             close_files = Arc::try_unwrap(mem::take(&mut context.files))
                 .map_or_else(|_| Vec::new(), RwLock::into_inner);
-            addrspace_opt =
-                mem::take(&mut context.addr_space).and_then(|a| Arc::try_unwrap(a).ok());
+            addrspace_opt = context.set_addr_space(None).and_then(|a| Arc::try_unwrap(a).ok());
             drop(context.syscall_head.take());
             drop(context.syscall_tail.take());
             context.id