In the last kernel cycle we migrated most of the `core::ffi` cases in
commit d072acda4862 ("rust: use custom FFI integer types"):

    Currently FFI integer types are defined in libcore. This commit
    creates the `ffi` crate and asks bindgen to use that crate for FFI
    integer types instead of `core::ffi`.

    This commit is preparatory and no type changes are made in this
    commit yet.

Finish now the few remaining/new cases so that we perform the actual
remapping in the next commit as planned.

Acked-by: Jocelyn Falempe <jfalempe@redhat.com> # drm
Link: https://lore.kernel.org/rust-for-linux/CANiq72m_rg42SvZK=bF2f0yEoBLVA33UBhiAsv8THhVu=G2dPA@mail.gmail.com/
Link: https://lore.kernel.org/all/cc9253fa-9d5f-460b-9841-94948fb6580c@redhat.com/
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

This adds a simple seq file abstraction that lets you print to a seq
file using ordinary Rust printing syntax.

An example user from Rust Binder:

    pub(crate) fn full_debug_print(
        &self,
        m: &SeqFile,
        owner_inner: &mut ProcessInner,
    ) -> Result<()> {
        let prio = self.node_prio();
        let inner = self.inner.access_mut(owner_inner);
        seq_print!(
            m,
            "  node {}: u{:016x} c{:016x} pri {}:{} hs {} hw {} cs {} cw {}",
            self.debug_id,
            self.ptr,
            self.cookie,
            prio.sched_policy,
            prio.prio,
            inner.strong.has_count,
            inner.weak.has_count,
            inner.strong.count,
            inner.weak.count,
        );
        if !inner.refs.is_empty() {
            seq_print!(m, " proc");
            for node_ref in &inner.refs {
                seq_print!(m, " {}", node_ref.process.task.pid());
            }
        }
        seq_print!(m, "\n");
        for t in &inner.oneway_todo {
            t.debug_print_inner(m, "    pending async transaction ");
        }
        Ok(())
    }

The `SeqFile` type is marked not thread safe so that `call_printf` can
be a `&self` method. The alternative is to use `self: Pin<&mut Self>`
which is inconvenient, or to have `SeqFile` wrap a pointer instead of
wrapping the C struct directly.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20241001-seqfile-v1-1-dfcd0fc21e96@google.com
Signed-off-by: Christian Brauner <brauner@kernel.org>