rust: sync: atomic: Implement Debug for Atomic<Debug>

If `Atomic<T>` is `Debug` then it's a `debugfs::Writer`, therefore make
it so since 1) debugfs needs to support `Atomic<T>` and 2) it's rather
trivial to implement `Debug` for `Atomic<Debug>`.

Tested-by: David Gow <davidgow@google.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>