Refactor kernel virtual memory allocation for kernel stack and I/O memory

ostd/src/lib.rs

@@ -3,6 +3,7 @@
 //! The standard library for Asterinas and other Rust OSes.
 #![feature(alloc_error_handler)]
 #![feature(allocator_api)]
+#![feature(btree_cursors)]
 #![feature(const_ptr_sub_ptr)]
 #![feature(const_trait_impl)]
 #![feature(core_intrinsics)]
@@ -90,13 +91,13 @@ unsafe fn init() {
 
     smp::init();
 
-    bus::init();
-
     // SAFETY: This function is called only once on the BSP.
     unsafe {
         mm::kspace::activate_kernel_page_table();
     }
 
+    bus::init();
+
     arch::irq::enable_local();
 
     invoke_ffi_init_funcs();