rust: drm: gem: shmem: Add DRM shmem helper abstraction

rust: drm: gem: shmem: Add DRM shmem helper abstraction

The DRM shmem helper includes common code useful for drivers which
allocate GEM objects as anonymous shmem. Add a Rust abstraction for
this. Drivers can choose the raw GEM implementation or the shmem layer,
depending on their needs.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Janne Grunau <j@jananu.net>
Tested-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Link: https://patch.msgid.link/20260316211646.650074-6-lyude@redhat.com
[ * DRM_GEM_SHMEM_HELPER is a tristate; when a module driver selects it,
    it becomes =m. The Rust kernel crate and its C helpers are always
    built into vmlinux and can't reference symbols from a module,
    causing link errors.

    Thus, add RUST_DRM_GEM_SHMEM_HELPER bool Kconfig that selects
    DRM_GEM_SHMEM_HELPER, forcing it built-in when Rust drivers need it;
    use cfg(CONFIG_RUST_DRM_GEM_SHMEM_HELPER) for the shmem module.

  * Add cfg_attr(not(CONFIG_RUST_DRM_GEM_SHMEM_HELPER), expect(unused))
    on pub(crate) use impl_aref_for_gem_obj and BaseObjectPrivate, so
    that unused warnings are suppressed when shmem is not enabled.

  * Enable const_refs_to_static (stabilized in 1.83) to prevent build
    errors with older compilers.

  * Use &raw const for bindings::drm_gem_shmem_vm_ops and add
    #[allow(unused_unsafe, reason = "Safe since Rust 1.82.0")].

  * Fix incorrect C Header path and minor spelling and formatting
    issues.

  * Drop shmem::Object::sg_table() as the current implementation is
    unsound.

    - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>