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 d969d50 ("rnull: enable configuration via
`configfs`") and commit 34585dc ("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>

Author: tamird

drivers/block/rnull/configfs.rs

@@ -1,12 +1,13 @@
 // SPDX-License-Identifier: GPL-2.0
 
 use super::{NullBlkDevice, THIS_MODULE};
-use core::fmt::{Display, Write};
 use kernel::{
     block::mq::gen_disk::{GenDisk, GenDiskBuilder},
     c_str,
     configfs::{self, AttributeOperations},
-    configfs_attrs, new_mutex,
+    configfs_attrs,
+    fmt::{self, Write as _},
+    new_mutex,
     page::PAGE_SIZE,
     prelude::*,
     str::{kstrtobool_bytes, CString},
@@ -99,8 +100,8 @@ impl TryFrom<u8> for IRQMode {
     }
 }
 
-impl Display for IRQMode {
-    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+impl fmt::Display for IRQMode {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             Self::None => f.write_str("0")?,
             Self::Soft => f.write_str("1")?,