rust: alloc: use `kernel::fmt`

rust: alloc: 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 9def0d0a2a1c ("rust: alloc: add
Vec::push_within_capacity").

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://patch.msgid.link/20251018-cstr-core-v18-6-9378a54385f8@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>