Use #[expect(lint)], not #[allow(lint)]

ostd/src/boot/smp.rs

@@ -31,7 +31,7 @@ struct PerApInfo {
     // no longer be used, and the `Segment` can be deallocated (this problem also
     // exists in the boot processor, but the memory it occupies should be returned
     // to the frame allocator).
-    #[allow(dead_code)]
+    #[expect(dead_code)]
     boot_stack_pages: Segment<KernelMeta>,
 }