rust: debugfs: Use kernel Atomic type in docs example

rust: debugfs: Use kernel Atomic type in docs example

Switch the read_callback_file() documentation example from
core::sync::atomic::AtomicU32 to the kernel's Atomic because Rust
native atomics are not allowed to use in kernel.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://patch.msgid.link/20251203000411.30434-1-fujita.tomonori@gmail.com
[ Use kernel vertical import style. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>