rust: add UnsafePinned type

This version now only has the kernel implementation without the
config flag for using the upstream version. Additionally now
commits for using `UnsafePinned` in `Opaque` were added.

Checkpatch warns about `rust/kernel/types/unsafe_pinned.rs`
missing a MAINTAINERS entry, I don't think that is necessary since it
will be part of the `RUST` entry anyways (from what I understand).

Once this has had some time to review I'll rebase my `miscdevice`
patches [0] on top of this.

This patchset depends on the `pin-init` sync for v6.16 [1].

Link: https://lore.kernel.org/rust-for-linux/20250131-b4-rust_miscdevice_registrationdata-v2-0-588f1e6cfabe@gmail.com/ [0]
Link: https://lore.kernel.org/rust-for-linux/20250421221728.528089-1-benno.lossin@proton.me [1]

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To: Sky <sky@sky9.dev>
To: Miguel Ojeda <ojeda@kernel.org>
To: Alex Gaynor <alex.gaynor@gmail.com>
To: Boqun Feng <boqun.feng@gmail.com>
To: Gary Guo <gary@garyguo.net>
To: Björn Roy Baron <bjorn3_gh@protonmail.com>
To: Benno Lossin <benno.lossin@proton.me>
To: Andreas Hindborg <a.hindborg@kernel.org>
To: Alice Ryhl <aliceryhl@google.com>
To: Trevor Gross <tmgross@umich.edu>
To: Danilo Krummrich <dakr@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: rust-for-linux@vger.kernel.org
Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
---
Changes in v2:
- Expanded `UnsafePinned` documentation to describe differences
    with upstream rust implementation.
- Removed config flag for using upstream `UnsafePinned` type.
- Add patch implementing `Wrapper` for `Opaque`
- Add patch for using `UnsafePinned` internally in `Opaque`
- Link to v1: https://lore.kernel.org/r/20250418-rust_unsafe_pinned-v1-1-c4c7558399f8@gmail.com

--- b4-submit-tracking ---
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        "20250418-rust_unsafe_pinned-v1-1-c4c7558399f8@gmail.com"
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      "message-id: <20250421221728.528089-1-benno.lossin@proton.me>",
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Author: onestacked