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/[email protected] [1]

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To: Sky <[email protected]>
To: Miguel Ojeda <[email protected]>
To: Alex Gaynor <[email protected]>
To: Boqun Feng <[email protected]>
To: Gary Guo <[email protected]>
To: Björn Roy Baron <[email protected]>
To: Benno Lossin <[email protected]>
To: Andreas Hindborg <[email protected]>
To: Alice Ryhl <[email protected]>
To: Trevor Gross <[email protected]>
To: Danilo Krummrich <[email protected]>
To: Gerald Wisböck <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Christian Schrefl <[email protected]>
---
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/[email protected]

--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
{
  "series": {
    "revision": 2,
    "change-id": "20250418-rust_unsafe_pinned-891dce27418d",
    "prefixes": [],
    "history": {
      "v1": [
        "[email protected]"
      ]
    },
    "prerequisites": [
      "message-id: <[email protected]>",
      "base-commit: 39051ad"
    ]
  }
}

Author: onestacked