Put `PerCoreScheduler::current_task` into an `InterruptRefCell`

[mkroening]

PerCoreScheduler::current_task is currently an Rc<RefCell<Task>>. Accesses to the RefCell are currently explicitly without_interrupts:

kernel/src/scheduler/mod.rs

Lines 576 to 584 in 53536a5

	/// Remove a IO interface, which is named by the file descriptor
	pub fn remove_object(&self, fd: FileDescriptor) -> io::Result<Arc<dyn ObjectInterface>> {
	without_interrupts(|| {
	let current_task = self.current_task.borrow();
	let mut object_map = current_task.object_map.write();
	object_map.remove(&fd).ok_or(Errno::Badf)
	})
	}

fbc6044 mistakenly removed without_interrupts from PerCoreScheduler::get_object, resulting in spurious panics at

kernel/src/scheduler/mod.rs

Line 748 in 53536a5

let mut borrowed = self.current_task.borrow_mut();

It would be beneficial to use an InterruptRefCell instead of a RefCell to ensure interrupts are disabled.

Related: #1846