rust: debugfs: use `kernel::fmt`

rust: debugfs: use `kernel::fmt`

Reduce coupling to implementation details of the formatting machinery by
avoiding direct use for `core`'s formatting traits and macros.

This backslid in commit 40ecc49466c8 ("rust: debugfs: Add support for
callback-based files") and commit 5e40b591cb46 ("rust: debugfs: Add
support for read-only files").

Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Matthew Maurer <mmaurer@google.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://patch.msgid.link/20251018-cstr-core-v18-7-9378a54385f8@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>