Panic in Double Fault handler instead of looping endlessly

The advantage of panicking is that it causes a QEMU exit in integration tests.

blog/content/second-edition/posts/07-double-faults/index.md

@@ -78,8 +78,7 @@ lazy_static! {
 extern "x86-interrupt" fn double_fault_handler(
     stack_frame: &mut InterruptStackFrame, _error_code: u64)
 {
-    println!("EXCEPTION: DOUBLE FAULT\n{:#?}", stack_frame);
+    panic!("EXCEPTION: DOUBLE FAULT\n{:#?}", stack_frame);
-    loop {}
 }
 ```