rust: pin-init: internal: init: add escape hatch for referencing initialized fields

rust: pin-init: internal: init: add escape hatch for referencing initialized fields

The initializer macro emits mutable references for already initialized
fields, which allows modifying or accessing them later in code blocks or
when initializing other fields. This behavior results in compiler errors
when combining with packed structs, since those do not permit creating
references to misaligned fields. For example:

    #[repr(C, packed)]
    struct Foo {
        a: i8,
        b: i32,
    }

    fn main() {
        let _ = init!(Foo { a: -42, b: 42 });
    }

This will lead to an error like this:

    error[E0793]: reference to field of packed struct is unaligned
      --> tests/ui/compile-fail/init/packed_struct.rs:10:13
       |
    10 |     let _ = init!(Foo { a: -42, b: 42 });
       |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       |
       = note: this struct is 1-byte aligned, but the type of this field may require higher alignment
       = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
       = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
       = note: this error originates in the macro `init` (in Nightly builds, run with -Z macro-backtrace for more info)

This was requested by Janne Grunau [1] and will most certainly be used
by the kernel when we eventually end up with trying to initialize packed
structs.

Thus add an initializer attribute `#[disable_initialized_field_access]`
that does what the name suggests: do not generate references to already
initialized fields.

There is space for future work: add yet another attribute which can be
applied on fields of initializers that ask for said field to be made
accessible. We can add that when the need arises.

Requested-by: Janne Grunau <j@jannau.net>
Link: https://lore.kernel.org/all/20251206170214.GE1097212@robin.jannau.net
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <lossin@kernel.org>