rust: fs: add Kiocb struct

This adds a very simple Kiocb struct that lets you access the inner
file's private data and the file position. For now, nothing else is
supported.

Cc: Christian Brauner <brauner@kernel.org>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250822-iov-iter-v5-3-6ce4819c2977@google.com