rnull: use `kernel::fmt`

rnull: 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 d969d504bc13 ("rnull: enable configuration via
`configfs`") and commit 34585dc649fb ("rnull: add soft-irq completion
support").

Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://patch.msgid.link/20251018-cstr-core-v18-5-9378a54385f8@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>