rust: debugfs: Add support for read-only files

Extends the `debugfs` API to support creating read-only files. This
is done via the `Dir::read_only_file` method, which takes a data object
that implements the `Writer` trait.

The file's content is generated by the `Writer` implementation, and the
file is automatically removed when the returned `File` handle is
dropped.

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-v11-2-7d12a165685a@google.com


[ Fixup build failure when CONFIG_DEBUGFS=n. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>