rust: configfs abstractions

Add a safe Rust API that allows Rust modules to interface the `configfs`
machinery.

Add an example for the samples folder to demonstrate usage of the API.

Add a maintainer entry for the Rust configfs abstractions in the last patch, to
make it absolutely clear that I will commit to maintain these abstractions, if
required.

The series is a dependency of `rnull`, the Rust null block driver.
Please see [1] for initial `configfs` support in `rnull`.

[1] https://github.com/metaspace/linux/tree/9ac53130f5fb05b9b...

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
Changes in v5:
- Remove `as _` casts.
- Document `ID` type parameter of `AttributeOperations`.
- Add documentation at macro call sites in example.
- Add example expansion of `configfs_attrs!`.
- Move trait bound in `AttributeList::add`
- Improve safety requirement for `AttributeList::new`.
- Fix a copy/paste error in print in sample.
- Clarify use of `{}` for empty struct in sample.
- Improve documentation for `AttributeList`.
- Remove `kernel::` prefix from `container_of!` invocation.
- Reword safety comment in `get_group_data`.
- Correct commit message in relation to unstable feature additions.
- Use imperative language in commit messages.
- Consistently capitalize the word "Rust" in commit messages.
- Explain that "drop" in `GroupOperations::drop_item` is not related to Rust
  drop.
- Link to v4: https://lore.kernel.org/r/20250224-configfs-v4-0-9af9b5e6...

Changes in v4:
- Fix a build issue by depending on v18 of "rust: types: add `ForeignOwnable::PointedTo`"
- Link to v3: https://lore.kernel.org/r/20250218-configfs-v3-0-0e40c077...

Changes in v3:
- Allow trailing commas in invocation of `configfs_attrs!`.
- Use a more suitable C initialization function when initializing `Subsystem`.
- Split sample into separate patch.
- Add an inline example.

The remaining changes in this version are style fixes, documentation
improvements and typo fixes. They are enumerated below:
- Consolidate `paste` macro calls.
- Do not hard code page size in example.
- Remove prefix of `c_str!` in sample.
- Use a more descriptive variable name in `into_foreign`.
- Improve code formatting in macros invocations.
- Add comment related to null terminator in `configfs_attrs!`
- Move attributes below docstrings.
- Remove a rogue todo.
- Remove trait bound from struct definition `GroupOperationsVTable`.
- Remove `as _` casts.
- Remove `GroupOprations::Parent` associated type.
- General documentation improvements.
- Explicitly use `ArcBorrow` for `drop_item` parameter type.
- Add a comment describing expansion to a call to `Attribute::add`.
- Add a comment explaining bound check in `Attribute::add`.
- Link to v2: https://lore.kernel.org/r/20250207-configfs-v2-0-f7a60b24...

Changes in v2:
- Remove generalization over pointer type and enforce use of `Arc`.
- Use type system to enforce connection between `ItemType` and
  `Subsystem` or `Group`. Differentiate construction of vtables on this
  type difference.
- Move drop logic of child nodes from parent to child.
- Pick `ForeignOwnable::PointedTo` patch as dependency instead of
  including it here.
- Fix some rustdoc warnings.
- Use CamelCase for generic type parameter declaration.
- Destroy mutex in `Subsystem::drop`.
- Move `GroupOperationsVTable` struct definition next to implementation.
- Rebase on v6.14-rc1.
- Link to v1: https://lore.kernel.org/r/20250131-configfs-v1-0-87947611...

---
Andreas Hindborg (4):
      rust: sync: change `<Arc<T> as ForeignOwnable>::PointedTo` to `T`
      rust: configfs: introduce rust support for configfs
      rust: configfs: add a sample demonstrating configfs usage
      MAINTAINERS: add entry for configfs Rust abstractions

 MAINTAINERS                     |    7 +
 rust/bindings/bindings_helper.h |    1 +
 rust/helpers/mutex.c            |    5 +
 rust/kernel/configfs.rs         | 1054 +++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs              |    2 +
 rust/kernel/sync/arc.rs         |   21 +-
 samples/rust/Kconfig            |   11 +
 samples/rust/Makefile           |    1 +
 samples/rust/rust_configfs.rs   |  192 +++++++
 9 files changed, 1289 insertions(+), 5 deletions(-)
---
base-commit: 379487e17ca406b47392e7ab6cf35d1c3bacb371
change-id: 20250131-configfs-b888cd82d84a
prerequisite-patch-id: 03c1bff48fd24e83d1383e98f8668300e665ebae

Best regards,
-- 
Andreas Hindborg <a.hindborg@kernel.org>