rust: sync: use `kernel::{fmt,prelude::fmt!}`

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

Acked-by: Greg Kroah-Hartman <[e-mail]>
Reviewed-by: Alice Ryhl <[e-mail]>
Reviewed-by: Benno Lossin <[e-mail]>
Signed-off-by: Tamir Duberstein <[e-mail]>
Signed-off-by: Miguel Ojeda <[e-mail]>

rust/kernel/sync/arc.rs

···
     alloc::{AllocError, Flags, KBox},
     bindings,
     ffi::c_void,
+    fmt,
     init::InPlaceInit,
     try_init,
     types::{ForeignOwnable, Opaque},
···
 use core::{
     alloc::Layout,
     borrow::{Borrow, BorrowMut},
-    fmt,
     marker::PhantomData,
     mem::{ManuallyDrop, MaybeUninit},
     ops::{Deref, DerefMut},