rust: debugfs: Add support for writable files

Extends the `debugfs` API to support creating writable files. This
is done via the `Dir::write_only_file` and `Dir::read_write_file`
methods, which take a data object that implements the `Reader`
trait.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250904-debugfs-rust-v11-3-7d12a165685a@google.com


[ Fix up Result<()> -> Result. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>