rust: add UnsafePinned type and use it in Opaque

I've finally got some time to work on linux again, so I'm resending this
patchset. I've updated it to match the upstream changes and added patches
to use it in `DevRes`.  

This patchset is based on the v6.18 tag.

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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>
To: Gerald Wisböck <gerald.wisboeck@feather.ink>
To: Nathan Chancellor <nathan@kernel.org>
To: Nick Desaulniers <nick.desaulniers+lkml@gmail.com>
To: Bill Wendling <morbo@google.com>
To: Justin Stitt <justinstitt@google.com>
Cc: linux-kernel@vger.kernel.org
Cc: rust-for-linux@vger.kernel.org
Cc: llvm@lists.linux.dev
Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
---
Changes in v5:
- Document that `UnsafePinned` now include `UnsafeCell` semantics.
- Update `UnsafePinned::get` docs and signature for `UnsafeCell` semantics.
- Rename `UnsafePinned::raw_get_mut` to `UnsafePinned::raw_get` to match
  Upstream.
- Added a patch to also use this in `DevRes`.
- Dropped Sky's Co-Authored-By since some upstream changes from other
  authors are now also included.
- Rebase ontop of v6.18
- Link to v4: https://lore.kernel.org/r/20250511-rust_unsafe_pinned-v4-0-a86c32e47e3d@gmail.com

Changes in v4:
- Reworked `UnsafePinned::add` doctest to not use get_mut_unchecked.
- Link to v3: https://lore.kernel.org/r/20250510-rust_unsafe_pinned-v3-0-57ce151123f9@gmail.com

Changes in v3:
- Dropped CONFIG_RUSTC_HAS_UNSAFE_PINNED and feature(unsafe_pinned) (Alice)
- Add comment to `Opaque` reasoning about included `UnsafeCell` (Benno)
- Small changes in commit message of Patch 3 (Benno)
- Removed docs mentioning not included functions (Benno)
- Removed docs mentioning implementation details and added that as
    comment instead (Benno)
- Link to v2: https://lore.kernel.org/r/20250430-rust_unsafe_pinned-v2-0-fc8617a74024@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 ---
# This section is used internally by b4 prep for tracking purposes.
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Author: onestacked