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

rust: block: 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.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/Custom.20formatting/with/516476467
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>