The main challenge with defining `work_struct` fields is making sure
that the function pointer stored in the `work_struct` is appropriate for
the work item type it is embedded in. It needs to know the offset of the
`work_struct` field being used (even if there are several!) so that it
can do a `container_of`, and it needs to know the type of the work item
so that it can call into the right user-provided code. All of this needs
to happen in a way that provides a safe API to the user, so that users
of the workqueue cannot mix up the function pointers.

There are three important pieces that are relevant when doing this:

 * The pointer type.
 * The work item struct. This is what the pointer points at.
 * The `work_struct` field. This is a field of the work item struct.

This patch introduces a separate trait for each piece. The pointer type
is given a `WorkItemPointer` trait, which pointer types need to
implement to be usable with the workqueue. This trait will be
implemented for `Arc` and `Box` in a later patch in this patchset.
Implementing this trait is unsafe because this is where the
`container_of` operation happens, but user-code will not need to
implement it themselves.

The work item struct should then implement the `WorkItem` trait. This
trait is where user-code specifies what they want to happen when a work
item is executed. It also specifies what the correct pointer type is.

Finally, to make the work item struct know the offset of its
`work_struct` field, we use a trait called `HasWork<T, ID>`. If a type
implements this trait, then the type declares that, at the given offset,
there is a field of type `Work<T, ID>`. The trait is marked unsafe
because the OFFSET constant must be correct, but we provide an
`impl_has_work!` macro that can safely implement `HasWork<T>` on a type.
The macro expands to something that only compiles if the specified field
really has the type `Work<T>`. It is used like this:

```
struct MyWorkItem {
    work_field: Work<MyWorkItem, 1>,
}

impl_has_work! {
    impl HasWork<MyWorkItem, 1> for MyWorkItem { self.work_field }
}
```

Note that since the `Work` type is annotated with an id, you can have
several `work_struct` fields by using a different id for each one.
Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>