rust: add UnsafePinned type and use it in Opaque

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. This should be fine because it
will be included in the RUST entry.

This patchset is based on the rust-6.16 tag.

# Describe the purpose of this series. The information you put here
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# You can add trailers to the cover letter. Any email addresses found in
# these trailers will be added to the addresses specified/generated
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# modified.

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.
- 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.
{
  "series": {
    "revision": 5,
    "change-id": "20250418-rust_unsafe_pinned-891dce27418d",
    "prefixes": [],
    "history": {
      "v1": [
        "20250418-rust_unsafe_pinned-v1-1-c4c7558399f8@gmail.com"
      ],
      "v2": [
        "20250430-rust_unsafe_pinned-v2-0-fc8617a74024@gmail.com"
      ],
      "v3": [
        "20250510-rust_unsafe_pinned-v3-0-57ce151123f9@gmail.com"
      ],
      "v4": [
        "20250511-rust_unsafe_pinned-v4-0-a86c32e47e3d@gmail.com"
      ]
    },
    "prerequisites": []
  }
}

Author: onestacked