`<Task<T> as ArcWake>::wake` might sleep while it couldn't

[nbdd0121]

If the FIXME line in rust/kernel/kasync/executor/workqueue.rs in #958 is removed, klint gives the following error:

error: trait method annotated to have preemption count expectation of 0..
   --> rust/kernel/kasync/executor/workqueue.rs:147:5
    |
147 |     fn wake(self: Arc<Self>) {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: but the expectation of this implementing function is 0
note: the trait method is defined here
   --> rust/kernel/kasync/executor.rs:73:5
    |
73  |     fn wake(self: Arc<Self>) {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^
note: which may drop type `kernel::sync::Arc<kernel::kasync::executor::workqueue::Task<core::future::from_generator::GenFuture<[static generator@samples/rust/rust_echo_server.rs:25:75: 31:2]>>>` with preemption count 0..
   --> rust/kernel/kasync/executor/workqueue.rs:149:5
    |
147 |     fn wake(self: Arc<Self>) {
    |             ---- value being dropped is here
148 |         Self::wake_by_ref(self.as_arc_borrow());
149 |     }
    |     ^
note: which may call this function with preemption count 0..
   --> rust/kernel/sync/arc.rs:236:5
    |
236 |     fn drop(&mut self) {
    |     ^^^^^^^^^^^^^^^^^^
note: which may drop type `kernel::sync::arc::ArcInner<kernel::kasync::executor::workqueue::Task<core::future::from_generator::GenFuture<[static generator@samples/rust/rust_echo_server.rs:25:75: 31:2]>>>` with preemption count 0..
   --> rust/kernel/sync/arc.rs:255:22
    |
255 |             unsafe { core::ptr::drop_in_place(inner) };
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: which may drop type `kernel::kasync::executor::workqueue::Task<core::future::from_generator::GenFuture<[static generator@samples/rust/rust_echo_server.rs:25:75: 31:2]>>` with preemption count 0..
    = note: which may drop type `kernel::sync::Arc<kernel::kasync::executor::workqueue::Executor>` with preemption count 0..
note: which may call this function with preemption count 0..
   --> rust/kernel/sync/arc.rs:236:5
    |
236 |     fn drop(&mut self) {
    |     ^^^^^^^^^^^^^^^^^^
note: which may drop type `kernel::sync::arc::ArcInner<kernel::kasync::executor::workqueue::Executor>` with preemption count 0..
   --> rust/kernel/sync/arc.rs:255:22
    |
255 |             unsafe { core::ptr::drop_in_place(inner) };
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: which may drop type `kernel::kasync::executor::workqueue::Executor` with preemption count 0..
    = note: which may drop type `kernel::Either<kernel::workqueue::BoxedQueue, &kernel::workqueue::Queue>` with preemption count 0..
    = note: which may drop type `kernel::workqueue::BoxedQueue` with preemption count 0..
note: which may call this function with preemption count 0..
   --> rust/kernel/workqueue.rs:433:5
    |
433 |     fn drop(&mut self) {
    |     ^^^^^^^^^^^^^^^^^^
    = note: but this function expects preemption count 0

this can happen is a Waker is called after the executor is dropped (and the tasks cancelled).